Aristotle’s Qualitative Physics

The goal of this subproject is to understand what Aristotle’s qualitative physics means for the project of the De anima, and particularly for its account of perception.

It is one of TIDA’s overarching aims to resituate the project of Aristotle’s De anima (An.) without severing genuine points of contact between it and the aims and concepts of contemporary philosophy of mind. There could hardly be any terrain more in need of careful mapping than the points of continuity and discontinuity between Aristotle’s views on perception and contemporary thinking about perception. Plants have souls and are alive for Aristotle, yet nobody would say that Aristotle thinks they have minds. It is perception that makes an animal an animal for Aristotle and distinguishes animals from plants, and so it is rather with his views on perception that contact with the interests of contemporary philosophy of mind begins. Nor is perception just one cognitive capacity amongst others in this respect: it is well-known that Aristotle draws on his account of perception to develop his account of thought. It follows that one could not hope to understand Aristotle’s views on thought without first coming to terms with his views on perception. Understanding Aristotle’s views about perception is central to the task of accurately locating him with respect to the concerns of contemporary philosophy of mind.

How does Aristotle propose to study perception? According to the methodology outlined in An. II.4, we should define the capacity for perception in terms of its corresponding activity — perceiving. In turn, however, Aristotle suggests that we should define the activity of perceiving in terms of its corresponding object: ‘the perceptible’ (aisthêton). In consequence, it turns out that from Aristotle’s point of view there is no adequate way of understanding what perception and perceiving are, without understanding ‘the perceptible’.

What is ‘the perceptible’? Answers to this question are rife with controversy. Nonetheless, it is widely agreed that the perceptible includes at least the traditional perceptible qualities in terms of which Aristotle defines the traditional five senses and distinguishes them from one another: colour, sound, and odour, etc. For just as his methodology demands, Aristotle defines vision in terms of the activity of seeing, and then seeing in terms of colour (An. II.7), and likewise with the triple of audition, hearing, and sound, etc.

So far, so good. Yet Aristotle claims in An. III.2 that the actuality of perception — the activity of perceiving — and the actuality of the perceptible are ‘one and the same’, and consequently that the perceptible is merely potential outside perception. Such remarks have led various commentators to question Aristotle’s commitment to the mind-independence of perceptible qualities. Berkeley goes the farthest, insisting that perceptible qualities have no mind-independent reality at all for Aristotle, and indeed that Aristotle is an idealist like himself (1744). Many recent commentators suggest instead, and more cautiously, that Aristotle wishes to avoid a ‘naïve realism’ about perceptible qualities (Shields 2016), or that he adopts a ‘subtle realism’ about them (Marmodoro 2018), and so on.

Yet how are remarks like these in An. III.2 to be squared with Aristotle’s methodology for studying perception in the De anima? His methodology seems to presuppose that perceptible qualities are definitionally prior to perception: that perception is defined in terms of perceptible qualities, but not the other way around. For how could colour (e.g.) give us any independent leverage in understanding vision and seeing, if it turned out that colour was defined in terms of them? More than that, Aristotle argues explicitly that perceptible qualities are prior to perception ontologically too: that perceptible qualities can exist without perception, but not vice-versa (cf. Cat. 7). In short, it seems that whatever we say about An. III.2, we have to accommodate the fact that Aristotle’s whole approach to perception in the De anima seems to presuppose a robust realism about perceptible qualities like colours, sounds, and odours.

This brings us to Aristotle’s so-called ‘qualitative physics’ as it pertains to the project of explaining perception in the De anima. When commentators are not worrying about passages like An. III.2, it is widely agreed that Aristotle’s physics is not just quantitative, but qualitative too in the sense that he treats qualities like colours, sounds, and odours as perfectly respectable, causally efficacious, features of the natural world, on all fours in this respect with properties like motion and extension. This makes for a striking contrast between Aristotle’s physics and the standard physics today, since we are heirs to the purely quantitative physics developed in the early modern period and in explicit contrast with Aristotle’s physics. Indeed, when Hume wishes to contrast the philosophy of his day with that of Aristotle, he points to precisely this difference: ‘The fundamental principle of [the modern] philosophy is the opinion concerning colours, sounds, tastes, smells, heat and cold; which it asserts to be nothing but impressions in the mind, derived from the operation of external objects, and without any resemblance to the qualities of the objects’ (Treatise, I.IV.4).

We saw above that Aristotle wishes to define vision in terms of colour, and audition in terms of sound, etc. If Aristotle does indeed have a qualitative physics, then he is defining vision and audition in terms of features that are unlike anything recognized in today’s physics. This means on the one hand that we have to be very careful in comparing Aristotle’s conception of perception with our own. Yet it also means on the other hand that we can expect his conception of perception to be sufficiently different from our own that it will provide a fresh perspective from which to think both about perception, and our own preconceptions about it.