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Ibn Sīnā and Descartes on the Nature of Body

Thus, a body is such an entity that, if one posits a longitude on it, another longitude will be found intersecting it at a right angle, and a third longitude of these two lengths will stand as a perpendicular on the point of the previous intersection. Whatever can be placed under these three magnitudes in the aforesaid manner and is also a substance is called a body ... But that which is in a body, such as length, width, and depth, is known to exist not in the form of the body, but as an accident to it. For instance, one can take a piece of wax and elongate it to make it one hand longer, two fingers wider, one finger deeper. Thereafter one can modify it so that its length width and depth vary. Under such circumstances its bodily form will always persist, whereas these three dimensions do not persist. Thus, these three dimensions are accidents to the wax, while its form is another attribute. Bodies differ not with respect to form because, by belonging to one kind of category, all bodies are identical with respect to the possibility of being described by these three dimensions in the aforesaid manner.

The Metaphysica of Avicenna (ibn Sīnā), tr. Morewedge (1973), ch. 4

In a famous passage of the Second Meditation, Descartes asks us to consider a piece of wax melting on a stove. According to Descartes, as the wax undergoes this process of melting, every feature of the wax detectable by the senses changes, but the wax continues to exist. At this stage in the Meditations, we are still in doubt about whether the wax really exists. However, whether it exists or not, Descartes argues, the fact that I am capable of thinking of the wax as persisting through these changes shows that I have a concept of the wax itself which cannot be identified with anything revealed by the senses. Descartes ultimately argues that this conception of the wax itself is nothing but the concept of extended substance, i.e., of a thing which has some length, width, and breadth or other. This notion is linked to geometry, a science undertaken by the pure intellect and not by the senses.

For Descartes, this argument is a step on the way to a defence of mechanism, the thesis that bodies have no intrinsic features other than the modes of extension (ways of being extended) and interact only by collision.¹

Ibn Sīnā's discussion of the nature of body at the beginning of Metaphysica² is, in a number of respects, interesting to set alongside Descartes. It is not so surprising that ibn Sīnā and Descartes both make the argument for distinguishing a substance from its modes/accidents by treating the substance as that which persists through a change in the accidents—presumably they both got this from Aristotle's Categories. It is more striking that ibn Sīnā, like Descartes, uses a piece of wax as his example. It would be interesting to trace the chain of influence here. Morewedge (the translator) notes some sort of similar uses in Plato's Theatetus and Aristotle's Physics. To me, however, the most interesting feature of the discussion is that ibn Sīnā, like Descartes, holds that extension alone constitutes the essence/nature/form of body. Furthermore, ibn Sīnā goes on to argue that this form is identical with the body itself:

the substratum of a material form is not an actuality without a material form. It is an actual substance due to the material form. In reality, therefore, the material form is the substance ... Furthermore, the material substratum is by itself not a thing without a material form. It is impossible for reason to understand the description of the substance without this necessary accident. (ch. 8)

This seems to me to mirror Descartes's doctrine that there is only a conceptual distinction, and not a real distinction, between a substance and its principal attribute (e.g., between a body and extension).

However, despite this robust agreement on foundational issues regarding the nature of body, ibn Sīnā does not turn out to be a mechanist, and this reveals that Descartes needs an additional premise which is much less explicit in Meditations than his claims about the nature of body. Ibn Sīnā avoids drawing the mechanist conclusion because he accepts the Aristotelian doctrine of real accidents distinct from, but residing in and depending on, the substance:

If we suppose that quality, such as whiteness or blackness ... stood by itself, and did not depend on anything else, and did not partake of division, then neither blackness nor whiteness could exist ... A body is that which is divisible since this receptivity to division is the meaning of a body. Hence, it can be both white and black (i.e. at different times it can contain contrary characteristics).³ The peculiarity of whiteness or blackness is different from the meaning of being a body, which admits no contrary. Being black is something other than being receptive to divisibility. Whereas being receptive to divisibility is the mark of a body, blackness is nothing but blackness itself. Consequently, blackness is dependent on the body, not independent of the body. (ch. 10)

The view here is that color cannot be reduced to anything in the nature of body (i.e., extension), but nevertheless cannot exist apart from body, since only an extended thing can be colored. This is clearly an anti-mechanist color realism. This, however, does not in any way contradict the view that extension constitutes the nature of body, unless one adds the further premise that there must be an intelligible relation between the nature of a substance and its modes/accidents. This further premise (which Donald Rutherford, discussing Leibniz's version, dubbed the Principle of Intelligibility) is indeed a core principle of Cartesian philosophy. Thus we can see that it is really Descartes's commitment to the thoroughgoing intelligibility of nature—and in particular to the idea that all states of substances can be explicated through their natures, which can be grasped by the pure intellect—is a central plank of his argument for mechanism, and is in fact the place where at least some Medieval Aristotelians want to get off the boat.

(Cross-posted at The Mod Squad.)

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Notes

1. Although I won't go into the evidence for this here, it seems to me that the overt agenda of the Meditations—securing our knowledge of the existence of God and the natural immortality of the soul—conceals a hidden agenda—selling mechanistic physics to the Catholic Church. Further, it seems to me that Descartes cares a great deal more about this hidden agenda than he does about the overt agenda. That's not necessarily to say he's insincere in his religious/theological assertions, but I don't think these are among his core interests or motivations the way they are for some other early modern philosophers.
2. I am not an expert on Medieval philosophy in general or Medieval Islamic philosophy in particular and am reading this work for the first time, in translation. Metaphysica appears to be a title given by the translator to the first (metaphysical) part of work called Dānish Nāma-i'alā'ī, a summary of Ibn Sīnā's philosophy written in the vernacular Persian language rather than the scholarly Arabic of his other works.
3. I think these parentheticals are the translator's additions.

Posted by Kenny at December 14, 2018 10:09 AM