Book 8
5
Now the substance must be either eternal or perishable without ever being in process of perishing, and generated without ever being in process of generation. It has been clearly demonstrated elsewhereCf. Aristot. Met. 7.8. that no one generates or creates the form; it is the individual thing that is created, and the compound that is generated.
6
But whether the substances of perishable things are separable or not is not yet at all clearCf. Aristot. Met. 8.1.6. n..; only it is clear that this is impossible in some cases, i.e. in the case of all things which cannot exist apart from the particular instances; e.g. house or implement.Cf. Aristot. Met. 7.8.6. Probably, then, neither these things themselves, nor anything else which is not naturally composed, are substances; for their nature is the only substance which one can assume in the case of perishable things.
7
Hence the difficulty which perplexed the followers of AntisthenesCf. Aristot. Met. 5.29.4. and others similarly unlearned has a certain application; I mean the difficulty that it is impossible to define what a thing is (for the definition, they say, is a lengthy formula), but it is possible actually to teach others what a thing is like; e.g., we cannot say what silver is, but we can say that it is like tin.
