PHYSICS changes between non-living substances, of which he would give as an example the turning of water into steam In such cases he uses the formal and material causes as explanatory principles Change, for Aristotle, could take place in many diVerent categories: growth, for instance, was change in the category of quantity, and a change in a quality (e.g of colour) was called an alteration (GC 320a13) Local motion, as we have seen, is change in the category of place But change in the category of substance, where there is a change from one kind of thing into another, was a very special kind of change When a substance undergoes a change of quantity or quality, the same substance remains throughout, with its substantial form But if one kind of thing turns into another, does anything remain throughout? Aristotle answers: matter We have a case of alteration when the subject of change is perceptible and persists, and merely changes its properties A body, for instance, while remaining the same body, is now healthy and now ill; some bronze may be now circular and now angular, and yet the same bronze But when nothing perceptible persists in its identity as a subject of change, and the thing changes as a whole (when e.g semen becomes blood, or water changes into air, or air totally into water), such an occurrence is a case of one substance coming to be and another substance ceasing to be Matter, in the most proper sense of the term, is to be identiWed with the underlying subject which is receptive of coming-to-be and passing away (GC 319b8–320a2) What is the nature of this matter that underlies substantial change? Aristotle constantly explains the relationship of matter to form in living things (e.g in the formation of a foetus, as he archaically described it above) by analogy with artefacts ‘As the bronze is to the statue, the wood is to the bed, or the formless before receiving form is to the formed object, so is the underlying nature to the substance’ (Ph 191a9–12) The analogy is not easy to grasp What is the underlying nature that remains through substantial change in the way in which wood remains wood before and after being made into a bed? Surely the reshaping of wood or bronze is an example of an accidental, not a substantial change Things not yet get any clearer when Aristotle tells us, By matter I mean what in itself is neither of any kind nor of any size nor describable by any of the categories of being For it is something of which all these things are predicated, and therefore its essence is diVerent from that of all the predicates All the other categories are predicated of substance, but substance 191