ABSTRACT
If we have things changing we must distinguish between some qualities that characterize a thing essentially and others that are applicable only per accidens. For in order to identify the thing before it changed and re-identify it again afterwards we must be able to describe it to some extent, so that both the speaker and the hearer can talk about the same thing. Therefore it must have some qualities that it has both before and after the change. But equally there must be some quality that it has before but not after or after but not before the change, in virtue of which it is said to have changed – that is, to have become different. The former qualities are said to be essential, the latter only “accidental”. The distinction between essential and accidental qualities need not be the same for all things, nor indeed need it be the same for one thing on all occasions. Nevertheless it would be nice if we could have one pervasive and permanent distinction.
