2.1 Al-Ghazālī (d. 505/1111)

This indirect way of critiquing the “rule of one” is clearly evinced in Avicenna’s chief critic, the AshÊ¿arite theologian AbÅ« Ḥāmid al-GhazālÄ« (d. 505/1111). The gist of al-Ghazālī’s objections to the ex uno principle in the Third Discussion of his Tahāfut al-falāsifa is that Avicenna cannot have it both ways: either he (a) commits to the “rule of one” steadfastly and denies the procession of any intrinsic forms of multiplicity so that the only things to ever proceed from the First are a series of eternal Intellects one after the other, each prior Intellect being the cause of the one after it¹⁹—an outcome Avicenna clearly rejects; or he accepts that either (b) some degree of multiplicity must be admitted in the First Itself,²⁰ or—contrary to his principle—(c) that composites proceed from the First directly. For al-GhazālÄ«, only the second and third options guarantee the possibility of non-intellectual substances such as bodies and souls coming into existence. Furthermore, given his own AshÊ¿arite stance on God and His attributes, it is not so much the “rule of one” that is being questioned in al-Ghazālī’s critique but rather the First’s simplicity. Indeed, for al-GhazālÄ« the principle falters precisely because it is questionable whether the subject (mawḍūʿ) of the principle—namely the One as Avicenna conceives It—even exists.²¹

At this juncture it is worth reminding ourselves that although Avicenna’s general aim in the Ilāhiyyāt (IX.4) is to explain how the entire cosmos proceeds from the First Principle, he nonetheless tackles this problem in two very distinct stages. In the first stage, Avicenna’s goal is to demonstrate why it is impossible for bodies, souls and material forms to proceed as direct effects of the First Principle. He does this by arguing from the very nature of the First Principle and Its simplicity and necessity, showing that only another simple being that is “like” the First proceeds directly from the One. In the second stage, Avicenna recognizes that the “simplicity” of the first effect cannot be identical in every respect to the First’s “simplicity,” and hence some degree of equivocation is inevitable. This is a crucial step, else no real multiplicity emerges. Avicenna acknowledges therefore that for the soul and body of the outermost sphere to be generated, some degree of multiplicity perforce must be introduced at the level of the first effect in such a way that this is not itself the reflection of a prior multiplicity in the First Principle. Otherwise, he either confutes his own ex uno principle or inadvertently denies the First being a true One.

Avicenna’s solution to this delicate conundrum is to posit multiple acts of intellection (taÊ¿aqqul) or analytical reflections (iÊ¿tibārāt) in the first effect which, because the First does not think in a discursive fashion, cannot occur in the One. In this way, Avicenna introduces some element of multiplicity—albeit of an intellectual kind only—in the first effect (i.e. the Universal Intellect), which he emphatically states has no corresponding equivalent in the First Principle. It is through the productive quality of these intellectual acts that successive orders of multiplicity acquire their existence. Accordingly, the First Intellect performs three acts of intellection. By reflecting on its principle, the Necessary Existent, the First Intellect engenders the existence of the Second Intellect, and by reflexively thinking itself as both (a) necessary of existence by another and (b) possible of existence in itself, it bequeaths the existence of the first celestial soul and body, respectively.²²

The point of saying this is to highlight that out of the five objections raised by al-Ghazālī to Avicenna’s emanationist cosmology, four are concerned with Avicenna’s second purpose in Ilāhiyyāt IX.4 and as such do not target the “rule of one” directly. Al-Ghazālī is thus mainly concerned with Avicenna’s account of how successive orders of multiplicity emerge after the first effect. By showing that the Avicennan account of how lower beings come into existence is defective, al-Ghazālī has enough ammunition to target the procession of existence from the First Principle Itself. The argument focusses on the nature of simplicity and composition and whether “multiple” intellections in the first effect constitute an instance of intrinsic multiplicity. If, as Avicenna holds, the emergence of the Second Intellect, the celestial soul and body of the outermost sphere require three acts of intellection in the First Intellect, then the simple fact that the products issuing from these acts are discrete existences is enough to convince al-Ghazālī that the corresponding acts of intellection performed by the first effect cannot all be one and the same in extenso. That is, the First Intellect’s thinking itself cannot be the same act of thinking as when it thinks the First Principle, for example. But if on the contrary all three acts of intellection amount to just a single act, then on what basis can Avicenna claim that three distinct existences emerge from one act of intellection by the First Intellect? Is this not itself a clear instance of Avicenna subverting his own “rule of one”?

What al-GhazālÄ« is ultimately gesturing here is that if the first effect’s three acts of intellection are all one and the same but nonetheless sufficient to cause the generation of the many, then why posit the existence of intermediary intellects in the first place? For surely God Himself in whom the subject (al-ʿāqil), act (al-Ê¿aql), and object (al-maÊ¿qÅ«l) of intellection are identical—as Avicenna believes—suffices for the procession of composite beings directly.²³ Al-GhazālÄ« thus questions Avicenna’s consistency, for it seems the latter has one rule for God and another rule for the First Intellect.²⁴

2.2 Sharaf al-Dīn al-Masʿūdī (fl. 582/1186)

Heavily influenced by al-GhazālÄ« and AbÅ« l-Barakāt al-BaghdādÄ«, the Bukhara scholar Sharaf al-DÄ«n al-MasÊ¿Å«dÄ« is the first post-Avicennan thinker to engage with the “rule of one” directly. Citing tanbÄ«h V.11 from the Ishārāt, al-MasÊ¿Å«dÄ« explains that Avicenna’s aim is “to explain how things [come into] existence from the First Principle in the order that is necessary for them, and how despite their generic, specific and individual differences they all terminate in a single Principle.”²⁵ But this aim, says al-MasÊ¿Å«dÄ«, cannot be fulfilled without two prior axioms of Avicenna’s metaphysics: (1) the principle that that which is One in every respect has only one thing proceed from it (i.e. the “rule of one”) and (2) that the Necessary Existent is one in every respect. As I have indicated above, al-MasÊ¿Å«dÄ« flatly rejects the second claim. As far as al-GhazālÄ« and al-MasÊ¿Å«dÄ« are concerned, the Avicennan One doesn’t exist.

Al-MasÊ¿Å«dÄ« grapples with the “rule of one” directly when he begins to focus on the first of his two mentioned axioms. He starts with a question directed not at the One from which things emanate, but the “one” that proceeds from the First Principle. “Is this ‘one,’ ” he writes, “one in species or one in number?” If it is one in number, then this implies it is simple and the only one of its kind, because there are no others like it. If it is one in species then it possesses a single nature, but this alone does not preclude others like it from existing also nor does it negate its being a divisible essence from another perspective.²⁶ Avicenna clearly regards the first effect to be one in number; al-MasÊ¿Å«dÄ« rejects this description of the proceeding “one.”

He writes,

If you say that the proceeding “one” is one in number only, then this cannot be admitted and there is no indication for it in what you have mentioned. For if A and B are two individuals belonging to a single species, then it is conceivable that both are necessitated from a single aspect. A difference in aspect and relation would only be incumbent if the two proceeding things differed in essence and nature.²⁷

This seems to miss Avicenna’s point entirely. Regardless of whether the effects belong to the same species or not, the fact remains that the notion of the First being a cause of A is different from the notion of It being a cause of B. So, evidently there are two aspects, not one.

If, on the other hand, the first effect is one in species, then al-MasÊ¿Å«dÄ« believes this claim can be confuted by the nature of being (wujÅ«d) itself. For being qua being is “one” inasmuch as every existent qua existence is the same; the only difference among existents is that they possess different essences. Therefore, just as the light which emanates from the Sun is the same regardless of how many rays proceed from it and how many objects receive this light, likewise the wujÅ«d that emanates from the First is the same irrespective of the inherent differences in the essences of the things receiving it. As al-MasÊ¿Å«dÄ« says, “the things [that receive existence] differ in terms of their natures (ḥaqāʾiq), not their existences, for they are caused things [only] inasmuch as their existences are caused and not their essences.”²⁸ Apart from begging the question as to where these essences came from to receive existence in the first place, al-MasÊ¿Å«dÄ« nevertheless raises an interesting point about the subject of instauration (jaÊ¿l) which, as we shall see, is discussed in a rather interesting way by Naṣīr al-DÄ«n al-ṬūsÄ«.

2.3 Fakhr al-Dīn al-Rāzī (d. 606/1210)

The Ashʿarite theologian Fakhr al-Dīn al-Rāzī discusses Avicenna’s “rule of one” in several of his writings. Of these, the al-Mabāḥith al-mashriqiyya is the most detailed and systematic. Al-Rāzī presents and critiques both sides of the debate in different sections of the book, and therefore provides a comprehensive treatment of the “rule of one” per se and the ensuing the question of how successive orders of multiplicity emerge after the first effect. Our focus in what follows is limited to the first discussion only. According to al-Rāzī, there have been four arguments that have been historically used as proofs for the metaphysical assertion that from the One only one proceeds. All four arguments make their way into later kalām discussions, and thus deserve to be considered one by one.

(I) The first is Avicenna’s “main” (Shifāʾ/Ishārāt) argument that whenever two things proceed simultaneously from the One, then either the One is not a simple reality or nothing at all proceeds from It given that an infinite series of simultaneous concomitant causes and effects (lawāzim) is impossible. What follows are six counterarguments by al-RāzÄ« against the conclusion of the first disjunct, namely, that every occurrence of multiplicity implies a composition in their original cause. Due to limited space, I shall consider just four of these.²⁹

In his first rebuttal, al-RāzÄ« begins by asserting that the center of a circle is a terminus for a potentially infinite number of radial lines, each beginning at the circumference and ending at its centre. The centre is also a point; and according to the First Definition in Book 1 of Euclid’s Elements, “A point is that which has no parts.” However, considering Avicenna’s argument, whenever multiple things proceed (or, as in this case, converge) from a single point of origin (or, as in this case, point of convergence) then the point in question cannot be simple. In fact, since there are a potentially infinite number of radial lines there must be a potentially infinite number of corresponding parts in the centre, which is absurd. Hence, Avicenna’s argument is false.³⁰

In the second rebuttal, he argues that if one were to take two simple realities A and B and consider them in the imagination jointly, then the notion of the set {A, B} that results from their conceptualization together would be different to our conceptualization of another set—e.g. {A, C}—in which a third simple reality is considered alongside A. Each set is conceptually different and distinct from the other, and by applying Avicenna’s own reasoning al-RāzÄ« concludes that all three simple realities would in fact have to be composite. But since the elements in question are all ex hypothesi simple in nature, Avicenna’s argument must be false.³¹

His third rebuttal also appears in his commentary on Avicenna’s Ishārāt and was the subject of an article published by Nicholas Heer more than twenty-five years ago.³² The objection here is that had the “rule of one” been valid, then the possibility of multiple negations from any simple being would also have to be denied. For example, our conceptualization that a certain thing is not a tree is different to our conceptualization that it is not a rock. Therefore, in the same way that the existence of multiple processions is either denied from the First simultaneously or, in the case that they do proceed from It, that the First is not a simple reality, likewise the case for multiple negations also: either they are impossible (in which case one cannot deny anything from the First) or the First is not a simple reality. Both options according to the falāsifa are false, and hence the “rule of one” must be false too.³³

The fourth and final objection is al-Rāzī’s claim that the notion of a thing being-a-cause (al-muʾaththiriyya) is a characterization of it by way of relation (iḍāfa) to something else. The point al-RāzÄ« is making here is that it is only after we conceive two things being side by side that we then recognize from their mutual interactions that one thing precedes the other in existence, and that it, viz. the prior existent, is the cause of the existent that came after it, viz. the effect. But “cause” and “effect” are mental correlatives and are not, therefore, intrinsic characterizations and determinations of the very essences of the things themselves. Hence, being-a-cause (al-muʾaththiriyya) and being-an-effect (al-muʾaththariyya) are descriptions belonging to the category of relation (maqÅ«lat al-iḍāfa). “And all [philosophers] concur,” says al-RāzÄ«, “that a multiplicity of relations does not lead to [the thing related] being multiple in essence.”³⁴

Out of the six rebuttals, this is perhaps the most powerful. Its full significance is immediately obvious when related back to the wording of the “rule of one” itself. The verb “to proceed” (ṣudūr) that is mentioned in the statement “From the One, only one proceeds” is by al-Rāzī’s reckoning a relative term, which characterizes how two things, A and B, are related to one another. We say, for example, that “B proceeds from A,” thereby making A the thing-from-which-another-thing-proceeds (al-maṣdar) and B the-thing-that-proceeds (al-ṣādir). However, since being-a-maṣdar and being-a-ṣādir are relations, they needn’t imply a composition in the very essence of that thing from which other things proceed, or for that matter in the proceeding things themselves. Hence, it is perfectly reasonable to posit multiple relations (and thus multiple processions) from the First without this impinging on Its simplicity. Underpinning this argument is the idea that relations (al-iḍāfāt) are analytical posits (iʿtibāriyya ʿaqliyya), not actual existential determinations. In other words, the characterization of A “being-a-maṣdar” and of B “being-a-ṣādir” are not real properties of either A or B, but rather mental considerations.

(II) The second proof that al-RāzÄ« mentions and then criticizes is Avicenna’s argument from the TaÊ¿lÄ«qāt. Avicenna argued that if the First causes the procession of A and B from a single aspect, then It simultaneously causes the procession of A and not-A along a single pathway, which he claims is a contradiction.³⁵ Avicenna’s wording is problematic. Logically speaking two propositions are contradictory when their qualities are different, i.e. one is affirmative and the other its negation. However, the propositions “F causes A” and “F causes not-A” are both affirmatives and thus conceivably false at the same time. Because of what appears to be a clear misunderstanding in Avicenna’s judgment, the passage provides a perfect opportunity for al-RāzÄ« to launch a scathing attack.

This kind of talk is so obviously wrong that even the weakest minds recognize it as being false. I have no idea, then, how something so obviously false has confused those who claim precise thinking. Indeed, it is baffling to me that someone [Avicenna] spends their entire life teaching the logical arts and studying them for the purpose of having a tool which prevents them from error, that when they are confronted with a matter of such noble importance they abandon its use and fall into error that even children mock at them.³⁶

As we will see below, not all later thinkers agree with al-Rāzī’s stringent reading of the Avicennan text despite its prima facie inconsistency.

(III) The third of al-Rāzī’s four arguments is one of the most important, historically speaking. Although I cannot elaborate on it in the depth it deserves here, it nonetheless has far-reaching consequences in later discussions on the nature of the one and the many. It is given special importance by mystically inclined philosophers such as Mullā Ṣadrā who, being inspired by the unitary metaphysics of Ibn ʿArabī’s commentators, utilize it to argue in favour of a kind of ontological continuity and existential symmetry between God and creation, thereby making the existence of the First Principle somehow preserved in, and continuous with, the existences of Its effects, albeit in Ṣadrā’s case through a metaphysics of “modulation” (tashkīk).

Al-RāzÄ« expounds on this view by using the word “harmonious” (lāʾim) to describe the close resemblance between effects and their causes. As he writes, “The cause must be harmonious with the effect, since each of us conceives a degree of harmony between fire and heat which we do not find between, say, water and heat.”³⁷ If at a general level it is true that every effect resembles its cause and is harmonious with it, the question arises what impact if any this has on the First Principle, which for argument’s sake causes two different realities from a single aspect. Accordingly, the First and Its two effects would all need to be harmonious. For al-RāzÄ«, this is nonsensical. How can the First be harmonious with Its effects and yet the effects themselves not be in harmony with each other? He begins the rebuttal by first clarifying that the notion that two things are harmonious just means that they are in some sense “similar” (mumāthil). In that case, the First is like Its effects in either a complete or partial way. If similar in a complete manner, then two absurd consequences follow: first, that neither the First nor what proceeds from It is more worthy of being regarded the cause of the other, but also—second—that two things which are in all respects similar and thus indiscernible are not two things at all but just one thing. On the other hand, if the First is like Its effect in some respects and not others, then this automatically suggests there are a multiplicity of aspects within the First, thus destroying Its simplicity. Hence, the First is neither similar nor harmonious with Its effect in any respect whatsoever.³⁸

(IV) In his final proof for our purposes, al-Rāzī explains the rationale behind Avicenna’s “rule of one” by making an analogy with observable phenomena. He begins the argument by considering a body that is first heated by fire and then cooled by water. The resulting rise and fall in temperature are described as two effects occurring to the body, one due to its contact with fire and the other because of its contact with water. Since the fire’s effect on the body is different from the water’s, it follows that the natures of fire and water cannot be the same; or else they would not have affected the body differently. Therefore, the falāsifa conclude that because the multiplicity of effects is a sign that their causes are different, wherever there are multiple effects there must be multiple causes.

For al-RāzÄ«, this argument should also be rejected because of its failure to distinguish the notions of ikhtilāf and takhalluf. The former signifies a numerical difference among individuals of the same species, while the latter signifies a difference among things due to a difference in their species. The upshot of the argument is that multiple effects entail multiple causes only when the effects are different by way of takhalluf, not ikhtilāf.³⁹ If Zayd laughs, Ê¿Amr cries, and Khālid learns to play the violin, then all these “effects” arise because of the fact they are all human beings, and thus a single nature suffices for multiple effects. Hence, there is no necessary entailment (mulāzama) between the effects being just numerically different and the inferred conclusion that their causes are multiple. More will be said about this argument below.

2.4 Naṣīr al-Dīn al-Ṭūsī (d. 672/1274)

In his commentary on Avicenna’s Ishārāt, the ShÄ«Ê¿Ä« philosopher Naṣīr al-DÄ«n al-ṬūsÄ« defends the Avicennan “rule of one” against each of his major critics from the previous century, from al-GhazālÄ« at the beginning all the way through to Fakhr al-DÄ«n al-RāzÄ« at the end. In other writings and personal correspondences composed later in his career however, especially those which were produced after the Mongol capture of Bagdad in 1256 and the beginning of his association with the Ilkhanids, al-ṬūsÄ« appears to take a more reticent view. For example, in the TajrÄ«d al-iÊ¿tiqād he describes each of the arguments advanced by the falāsifa to prove the existence of the intelligences (including Avicenna’s procession-argument) to be weak (madkhÅ«la),⁴⁰ though he does state elsewhere in the same work that there is equally no proof that they are impossible.⁴¹ I will start with the commentary on the Ishārāt and then consider some of his remarks in the TajrÄ«d. Needless to say, a full consideration of al-Ṭūsī’s opinions across all his writings is beyond the scope of this paper.

Al-ṬūsÄ« begins his comments on Avicenna’s tanbÄ«h (V.11) by first clarifying that the meaning of the key term “one” appearing on either side of the exemptive particle illā in the “rule of one” in fact means different things. As a description of the First Principle, “One” in the first occurrence (i.e. before illā) signifies the notion of a thing being essentially one, having no composition or multiplicity. As a description of the first effect (i.e. after illā), however, “one” signifies the notion of a thing being numerically one, thus designating it as the only instance of its kind in existence.⁴² In other words, the entire species of whatever is called numerically one is contained in its single instance—as happens to be the case, according to Avicenna, with respect to all the intelligences. By offering this initial clarification about the conceptual (sing. taá¹£awwur) distinction between each of the “ones” on either side of the procession principle, al-ṬūsÄ« replies to the question initially raised by al-MasÊ¿Å«dÄ« in his ShukÅ«k. This is an important step; without a correct taá¹£awwur of what the “rule of one” is saying, one cannot assent (taá¹£dÄ«q) to it in the way intended by Avicenna.

Al-ṬūsÄ« then turns to the actual argument itself. He agrees with Avicenna’s reasoning and states that if the First were a cause of two things, then the aspect of It being the cause of one is not the same conceptually as It being the cause of the other, and hence “the difference between these two ideas points to a difference in their two realities, implying that what was presumed to be [essentially] one is not in fact [essentially] one.”⁴³ Otherwise, says al-ṬūsÄ«, the First would either be two things in essence or something to which two different attributes are attached, in which case the being of the First is comprised of an essence and attributes. Both scenarios imply that the First is not a true One. By considering each of the permutations listed by Avicenna—namely that the effects proceeding from the First are either constituents (sing. muqawwim), concomitants (sing. lāzim), or a combination thereof—, al-ṬūsÄ« concludes by saying that “whenever two things result from something simultaneously, and neither is an intermediary for the other, then the thing from which they proceed is a divisible reality.”⁴⁴ Avicenna’s entire processionist cosmology rests on this conclusion. If correct, one can apply the rules of immediate inference to infer the proposition known as the “rule of one.” The conclusion obviously cuts two ways, however: either (1) one affirms the True One and denies It being the cause of simultaneous multiplicity (Avicenna and the falāsifa); (2) or one affirms the direct procession of the many qua many and denies the True One (al-GhazālÄ« and the AshÊ¿arite mutakallimÅ«n).⁴⁵

So, having explained the key terms and premises of Avicenna’s tanbīh, al-Ṭūsī turns his attention to the objections raised by Avicenna’s critics by first giving rejoinders to two of al-Rāzī’s objections. The first is a rejoinder to his predecessor’s objection in Sharḥ al-Ishārāt in which he argued that if Avicenna’s “rule of one” were correct, then it would also have to hold in the case of negations (“From the One, only one is negated”) and receptions (“To the One, only one is predicated”). However, this is clearly false. As for negations, the First is neither a tree, nor is It a rock. And as for predication, “a man is capable of sitting or standing” and “a body is either black or in motion” are both truthful statements, thus indicating the possibility of their subjects receiving multiple predicates. Hence, the “rule of one” is false.

From al-Ṭūsī’s perspective, the “noble commentator’s” counter-examples betray a clear lack of understanding. Indeed, as al-Ṭūsī remarks, each of al-Rāzī’s objections is metaphysically flawed for the simple fact that they each take for granted an already present and existent multiplicity. But since this isn’t the problem which Avicenna is concerned about, it seems al-Rāzī has missed the point entirely. One cannot explain the origins of multiplicity by seeking principles which themselves are many, since this merely begs the question. So, regardless of whether we take the examples he provides in Sharḥ al-Ishārāt or the example of the centre-point of a circle being the terminus of multiple radial lines in the Mabāḥith, they are all equally metaphysically flawed. Each presumes that many things already exist, and thus appear to miss the point of Avicenna’s “rule of one” which seeks to explain the origins of this many in the first instance.

Al-Ṭūsī combines all al-Rāzī’s objections and duly dismisses them in one fell swoop in a single passage:

The response is that (a) negating something from something, (b) predicating⁴⁶ something of something else, and (c) one thing being received by another—none of these is realized from the existence of one thing and one thing alone. None of them in fact results from something that is one qua one, but rather requires the existence of more than one thing preceding them, and hence from these [multiple] things [which are prior] these [other] things [which are posterior] follow through different considerations. Indeed, the procession of many things from many things is not impossible.⁴⁷

In his second rejoinder, al-ṬūsÄ« tackles an objection from al-Rāzī’s Mabāḥith. As shown above, al-RāzÄ« raised an important objection arguing that the notion of á¹£udÅ«r and its two participles maá¹£dar and ṣādir are relations, and as such are extrinsic descriptions of the First Principle and Its effect, respectively, which do not impinge on their essences directly, thereby allowing the possibility of multiple things proceeding from God without causing multiplicity in the essences of either of the correlatives. Al-ṬūsÄ« responds by clarifying that the word á¹£udÅ«r can be used in two different senses, either (1) as a relative term predicated extrinsically of causes and their effects when occurring together, just as al-RāzÄ« intimated; or (2) as an intrinsic term denoting the very nature and being of the cause itself, so that the cause—either as a whole due to its essence or because of a state occurring to it—is characterized as something from which a specific effect proceeds.⁴⁸ While admitting that the first meaning of á¹£udÅ«r is predicated posteriorly of each of the cause and effect’s existences, the second is a prior determination of the cause, characterizing it as a maá¹£dar irrespective of whether it is related to an effect or not. In other words, the second meaning of á¹£udÅ«r is predicated of the cause independently of the effect’s procession from it and before the effect is even existent. Al-Ṭūsī’s language is technical and convoluted, though crucial in terms of the Peripatetic system in which causes and effects are regarded as being essentially connected; i.e. the existence of the effect is already somehow contained in the nature of its cause and is a necessary consequence of it. Fire produces heat not simply because it has an accidental and haphazard relation to heat, but rather because the nature of fire is precisely such that given the right conditions it must necessarily produce heat. To deny this would in fact be a denial of secondary causation altogether, perhaps therein lying a clue as to the motivation of some of Avicenna’s AshÊ¿arite critics. As it appears in the “rule of one,” the term á¹£udÅ«r does not therefore refer to a relational property of the First and Its effect, but rather the very essence of the First Principle and Its first effect.

At this juncture it is worth shedding light on one of the most fiercely contested questions in the post-Avicennan tradition, already subtly alluded to by al-Masʿūdī in his Shukūk, concerning the precise causative relation a cause has to its effect in terms of the latter’s essence (māhiyya) and existence (wujūd). As is well known, at the core of Avicenna’s metaphysics is the idea that all contingent beings are composites of essence and existence. That being so, the question arises as to which of these is given to the effect by the cause in a primary and essential way in the process of “making” (jaʿl) the effect. Does the cause “make” the effect’s existence or its essence when it causes the effect? Logically speaking, there are only four options:

1. the effect’s wujūd is the thing-made (majʿūl);

2. the effect’s māhiyya is the thing-made;

3. both are made; or

4. neither is made.

The last of these possibilities is clearly impossible. If a cause neither makes its effect’s māhiyya nor its wujūd during the process of its instauration then it has not made the effect tout court. For many, the third option is just unfeasible given that it destroys the existential unity of extramental beings, not to mention the fact that it also leads to the problem of existence being superadded to quiddities in re and thus entails a well-known objection to the extramental supervening of existence. Therefore, the only viable options are (1) and (2). The importance of this debate can be immediately appreciated given the fact that the First Intellect is a contingent being and therefore a composite of essence and existence. This means that if both its essence and its existence were represented in re, then a simultaneous multiplicity would have proceeded from the First Principle, which although Avicenna states is impossible nonetheless presents a major challenge to the validity of the “rule of one.”

Al-Ṭūsī is acutely aware of this problem. What appears to have led him to consider it in the detail he has is perhaps instigated by his reading of al-Masʿūdī’s Shukūk, where there is a subtle hint that Avicenna’s procession principle may inadvertently backfire on Avicenna himself. If, as Avicenna believes, the First Intellect is a possible being having both quiddity and existence, and if, as Avicenna also believes, the mafhūm of a thing’s māhiyya is other than the mafhūm of its wujūd, then clearly one may ask whether this duality too emerges from the One Itself. Indeed, the very argument Avicenna provides to establish the “rule of one” in the Ilāhiyyāt of the Shifāʾ and in the Ishārāt could itself be applied in this instance to either undermine the simplicity of the First Principle or deny the procession of any first effect whatsoever. For Avicenna, both consequences would be catastrophic.

Al-Ṭūsī responds to this potentially devastating argument in a historically significant passage which although not appearing under the tanbīh in question, nonetheless addresses the problem at hand:

When something proceeds from the First Principle, that proceeding-thing has an ipseity (huwiyya) that is different from the ipseity of the First Principle.⁴⁹ As a result, the notion (mafhÅ«m) of “it” being something-proceeding from the First Principle is other than the notion of “it” being something possessing a certain identity. Hence, there are two intelligible meanings here: (a) the thing-emanating from the First, which is called existence (wujÅ«d); and (b) the identity [of this proceeding-thing] which is a concomitant of that existence called quiddity (māhiyya). This quiddity, insofar as existence [is concerned], follows that [original] existence [which emanated from the First Principle]. The reason for this is that if the First Principle did not produce anything then there would not have been any quiddity whatsoever [option (4) above]. However, conceptually speaking (fÄ« l-Ê¿aql) the existence [of the First Intellect] follows the quiddity [of the First Intellect in mental existence] given that existence is a [mental] property of quiddities.⁵⁰

That al-Ṭūsī has the problem of jaʿl in mind is confirmed in Quṭb al-Dīn al-Rāzī’s super-commentary. He explains that in al-Ṭūsī’s view, indicative of Avicenna’s view also, the existence of the contingent effect is what is originally made (majʿūl bi-l-dhāt) by the cause in the process of instaurating the effect; and that its quiddity is a follow-on concomitant of that existence and therefore only accidentally made (majʿūl bi-l-ʿaraḍ).

The popular view (al-madhhab al-mansÅ«b ilā l-qawm) is that quiddity is not made, but rather it is existence which is made. Therefore, it is existence which in reality proceeds from the First Principle. As for quiddity, its actualization in the external world is through [the actualization of] existence. Therefore, the quiddity [of the First Intellect] is an accidental act [mediated by the act of its existence], while the true act is the act of existence. If something proceeds from the [First] Principle, even though that thing shall have an ipseity, i.e. a quiddity, what really proceeds from It is existence. This is averred on the basis that quiddities are not made, and existence is different from quiddity […]. Existence and quiddity are thus two acts, except that one of them—namely, existence—is essentially made, while the other—namely, quiddity—is accidentally made.⁵¹

The “rule of one” also applies to the Avicennan essence-existence distinction, therefore. The two aspects of a contingent being proceed from their cause through different prior-posterior relations, and hence involve the procession of one thing through its intermediary. What proceeds directly from the cause is existence, and is thus—according to Mullā Ṣadrā’s nomenclature—the “fundamental” (aṣīl) ontic reality through which the quiddity of a thing is made. A lot more could and needs to be said here about the problem of jaʿl and its historical significance in the post-classical period, and about the precise meaning of the term iʿtibārī which is used to describe the nature of quiddity in later Islamic philosophy, especially in the thought of Mullā Ṣadrā. Suffice to say, our purpose here is merely to show that al-Ṭūsī had already anticipated this problem in terms of its relevance to the proof of the First Intellect. As far as the “rule of one” is concerned, only the existence of the First Intellect proceeds directly from the First Principle, and hence “From the One, only one proceeds.”

2.5 After al-Ṭūsī: A Brief Sketch

In his commentary on Ê¿Aḍud al-DÄ«n al-Ījī’s (d. 756/1355) al-Mawāqif, the Timurid theologian al-Sayyid al-SharÄ«f al-JurjānÄ« (d. 816/1413) observes that in order to derive the conclusion “From the One, only one proceeds,” Avicenna must first demonstrate that the converse of this statement is true. In other words, if he can demonstrate that multiplicity is always the outcome of multiplied causes, then Avicenna can easily apply the rules of immediate inference to derive his desired conclusion.⁵² To explain, let us recall that in each of his “proofs” for the procession principle, Avicenna needed to consider what impact, if any, a simultaneous procession of multiple effects from the One would have on the very nature of this First Principle. Since the argument is based on a proof by contradiction, Avicenna’s aim throughout each of his supporting arguments is to show that whenever multiple effects proceed simultaneously from the First, either by way of multiple aspects or just one aspect, then this always undermines the original claim that the First is a simple being. Hence, there is a contradiction in either (a) the nature of the First Itself or (b) in the effects proceeding from It. Since both outcomes lead to impossible conclusions, Avicenna can confidently assert that the original premise (i.e. the many proceeds from the One) is false; and hence by conversion can declare his desired conclusion, “From the One, only one proceeds.”

Avicenna’s ‘rule of one’ is thus derived from the following initial proposition:

P:	Whenever the effect is multiple, the cause is multiple (kullamā taʿaddada l-maʿlūl taʿaddadat al-ʿilla).

And by conversion:

C1:	Whenever the cause is not-multiple, the effect is not-multiple.

This can be re-worded as:

C2:	Whenever the cause is one, the effect is one (kullamā ittaḥada l-ʿilla ittaḥada l-maʿlūl).

If P is true, then C₁ and C₂ must be true also.

Al-ṬūsÄ« presented the same argument in his TajrÄ«d al-iÊ¿tiqād.⁵³ Given the significance of this work and its author’s reputation, the lemma of al-Ṭūsī’s argument stimulated myriad responses and debates in the commentaries and super-commentaries of the TajrÄ«d, thus making it a useful source for collating some of the different opinions on the procession principle by a host of different thinkers in the later medieval period. The first to criticize al-ṬūsÄ« was his student, the ShÄ«Ê¿Ä« mutakallim al-Ê¿Allāma al-ḤillÄ« (d. 726/1325). For al-ḤillÄ«, the proposition that simple causes produce single effects is a particular judgment applicable to causes acting by way of necessity only (bi-l-Ä«jāb) and not by free choice (bi-l-irāda), and thus cannot be generalized to cover all simple causes. While the procession principle may therefore in theory be applicable to the Necessary Existent in Avicenna’s theology, it is inapplicable to the God of traditional kalām, who unlike Avicenna’s First Principle is a volitional agent (fāʿil mukhtār). A freely choosing agent is thus excluded from the remit of the “rule of one” according to al-ḤillÄ«, who insists that the many proceed directly from the One and hence there is no reason to posit the existence of the First Intellect.⁵⁴ Like Fakhr al-DÄ«n al-RāzÄ«, al-ḤillÄ« also comments that the notions of á¹£udÅ«r and taʾthÄ«r are non-existential concepts, they are iÊ¿tibāriyya, and therefore do not cause the real multiplicity Avicenna fears would result if the First was a direct principle of multiple effects.⁵⁵ When, therefore, the notion of procession per se is an analytical consideration, a multiplicity of iÊ¿tibārÄ« processions would not undermine the First’s simplicity. Hence the original proposition (P) from which the procession principle is derived itself turns out to be false.

Another objection to the original lemma in al-Ṭūsī’s TajrÄ«d is identical to al-Rāzī’s objection in the Mabāḥith; namely, that the reason why a cause becomes multiplied in essence is not merely down to the fact that the effects proceeding from it are numerically multiple, but rather because they are essentially different. One cannot simply infer from the sheer numerical multiplicity of effects (ikhtilāf) that their cause is multiplied in essence (takhalluf).⁵⁶ As such, the mulāzama in Avicenna’s hypothetical argument is only valid when the effects are different in their essences, but not in that case when effects, although multiple in number, belong to the same species. Multiplicity per se cannot therefore be the true logical indicant (dalÄ«l) that the cause of each is itself multiple, and hence there is no truth—unqualifiedly speaking—to the claim that multiple effects are caused by multiple causes. Al-Ṭūsī’s fifteenth-century commentator Ê¿AlÄ« QÅ«shjÄ«, relying on al-Sayyid al-SharÄ«f al-JurjānÄ«, thus remarks as follows:

Supposing we were to see multiple effects without any [essential] difference [between them], still it would not be possible for us to infer the conclusion that the cause is multiple, for indeed that is the very thing which is being disputed.⁵⁷

To deflect objections of this kind against Avicenna’s Ishārāt and Shifāʾ arguments, one may in the end have to revert to Avicenna’s modified argument in the Taʿlīqāt. Although two effects in the same species are essentially the same, each would still be an instance (miṣdāq) of the other’s negation. Hence, the First would still have to be and not be the cause of a thing’s existence from a single aspect, which is still a contradiction. It is quite plausible therefore that Avicenna himself formulated the Taʿlīqāt argument as a response to exactly this kind of objection.

However, as we have seen, the argument is controversial and problematic. It is rejected by al-Rāzī—and before him Bahmanyār—on the basis that the propositions “F is a cause of A” and “F is a cause of not-A” are not exactly contradictory statements. In his commentary on al-Rāzī’s Mulakhkhaá¹£, Najm al-DÄ«n al-KātibÄ« (d. 675/1277) attacks Avicenna further by arguing that even if Avicenna were to be given the benefit of doubt, still his conclusion could not be granted. What if, al-KātibÄ« argues, Avicenna’s real intention was just the same as al-Rāzī’s own suggested corrective and that the contradictory of “F causes A” is “F does not cause A”? Is there now a contradiction? Al-KātibÄ« responds in the negative because neither proposition, he states, has a specified temporal quantifier, and so it is conceivable that both propositions are true albeit at different times.⁵⁸

In the fifteenth century, a response to these objections was given by the Timurid philosopher-theologian Jalāl al-DÄ«n al-DawānÄ« (d. 908/1502) in his glosses on al-QÅ«shjī’s commentary on al-Ṭūsī’s TajrÄ«d. Al-DawānÄ« observes that if the First Principle causes the procession of a not-A, and hence the proposition “F causes a not-A” is true, then it would still be correct to say, “F did not cause A” because A is not not-A. More exactly, al-DawānÄ« writes that the proposition “F causes not-A” is itself an instance of the proposition “F did not cause A” and thus, properly speaking, a contradictory of “F causes A.”⁵⁹ On this interpretation, al-DawānÄ« claims that Avicenna’s reasoning is perfectly accurate. As for al-Kātibī’s objection, this too according to al-DawānÄ« has no real bearing on the issue. Two apparently contradictory and unqualified propositions can still both be true if the times in which they are true are different, but false when the time in question is the same. However, as al-DawānÄ« points out, a discrepancy in times requires two different aspects, which therefore begs the question where this multiplicity has originated if the First Principle ex hypothesi has just a single aspect.⁶⁰ Furthermore, the context of the discussion here is with reference to an atemporal procession given that the Intellects, like their Principle, transcend time. Al-DawānÄ« is alluding to the fact that God and the First Intellect are pre-eternal, not in the sense that each is an everlasting being but rather that they transcend time altogether. Had Avicenna temporalized his propositions then he might have given the misleading impression that God and the First Intellect are in time and therefore temporal.

Avicenna’s procession argument is a powerful and enduring one, being recognized as such even by his detractors throughout the post-classical era. It calls attention to the fact that within the Neoplatonic contours of Avicenna’s philosophy the metaphysics of causation operates not only along an asymmetrical path, effects being always inferior to their causes, but also maintains a degree of proportionality between causes and their effects. From these initial premises, Avicenna fashions a metaphysically robust theory of how the many proceeds from the One. To do so, he employs a rich repertoire of metaphysical concepts and manoeuvres which are original to his own philosophy. Indeed, of all the historical arguments presented for the existence of the celestial Intellects in his oeuvre, the procession principle is the only one to proceed from first principles. Inspired by his Neoplatonic predecessors, Avicenna formalizes the concept of emanation through a rigorous form of systematic reasoning. Whereas Plotinus and other Arabic falāsifa had to rely on metaphor to express how things come into being from the One, Avicenna supplants linguistic imagery with robust logic, and thus revives the insights of the sages of old with renewed vigour. However, as I have tried to show in this paper, not everything was plain sailing. Avicenna’s arguments are open to criticism and appear to falter in important ways, which as a result stimulated intense debate and criticism. The conclusions he draws from the “rule of one” are clearly repugnant to the classical kalām tradition. Yet, despite the scathing criticisms of his early commentators, there was a burgeoning school of thought outside of kalām that embraced the Avicennan “rule of one,” all the while adapting it to fit in line with their mystically inclined worldview. Taking their inspiration from the Andalusian mystic Muḥyī l-Dīn Ibn ʿArabī (d. 638/1240), members of this school adapted the procession principle in accordance with their doctrine of the unity of existence (waḥdat al-wujūd), thereby envisioning new and interesting ways in which this principle could be used to support their version of a truly unitary metaphysics whereby the only thing to proceed from God, the Sublime Essence in Its Singularity (al-dhāt al-aḥadiyya), is “God” Himself. The entire cosmos is thus “God in manifestation” since, according to this mystical interpretation of Avicenna’s principle, “From ‘God’, only ‘God’ proceeds.” This, however, is a story for another occasion.