3.3 Aristotle's Physics

The earliest view of substances which gives a satisfactory account of potentialities and dispositions is that of Aristotle. We now know that some of the details of Aristotle's physics are wrong: bodies do not all tend to move to the centre of the earth, for example. But, as we explained earlier in the article, there is a philosophical approach that discusses general principles rather than specific knowledge of what actually happens. Although Aristotle's specific claims turned out to be incorrect, on a number of points his general principles are more satisfactory.   We may summarise his general principles as follows (following Gotthelf [1986], pp. 232 - 234). Nature, according to Aristotle, consists of individual entities, each of a specific kind, possessing various properties, moving and changing in various ways. They are all composed of simple bodies, the `elements', which are themselves analyzable into combinations of prime qualities and some sort of underlying matter. All natural things move and/or change in ways characteristic of themselves if not impeded.     That is to say, each has a nature, having `within themselves a source of motion-or-change and rest'^3.5 . A thing's nature explains these different characteristic changes. Other kinds of changes are caused by interactions, in which things act on other things.   Thus, in addition to having a nature, each natural thing has potentials to change certain other things in certain ways. Aristotle has no separate concept of `physical laws'. For him, explanations of particular changes are always in terms of the particular natures and/or potentials of the things involved. No appeal is made to some universal law, for each thing has within itself its nature and potentials, which are the source of changes to itself and others.

      Aristotle thought that there are only four elements, and that these are earth, air, fire, and water. These four are the different pair-wise combinations of the `prime qualities' dry/moist with hot/cold, so earth is dry and cold, water is cold and moist, air is moist and hot, and fire is hot and dry. It is not necessary for us, however, for us to accept this detailed identification even if his general principles seem sound. (We will see in chapter 9 later what may be more realistic identifications today.) What are relevant to our problem of substance, are his ideas of natures and potentials, which are those features of substances which lead them to behave as they do.  

  The problem with Newton's concept of substance, we can now see, is that nature of his corpuscles (as purely actual and definite atoms) does not lead to their gravitational attractions and their other dispositions such as their elasticity. If we described the nature of his corpuscles, we would know their position, velocity, size and mass. Because those are all their properties, they ought to tell us everything about how they can act and interact, but we still would not know about the existence or the strength of any gravitational attraction. Still less would we know about the electric, magnetic and nuclear attractions that have been discovered subsequently.