What comes to be either exists in itself, or in another. We use the English word “substance” to name what exists in itself as oppsed to existing in another, although when it is used in this sense it is used outside of modern English usage. I stick with the word here for historical reasons, and because of the lack of any good replacement (although “being” would come close). For something that exists in another, I use the word “accident”. Here, the modern English usage is close to the philosophical meaning: accidents are most familiar to us as events that take place in the context of things intended, but yet outside of the intention- we meant to do X, but while doingit, Y happened. Y is an “accident”. As I said, this meaning is close, but not exact.
The primary example of a substance in the sense I use it here is myself. For you, it is yourself. Said generally, a substance is best exemplified by John, Peter, Mary, etc. or any other individual of the species “man”. From this interior experience of our own existence and unity as selves, we can extend the meaning of self to include others. How far we can extend it is sometimes clear, other times unclear.
An accident, which is a way of existing in another, can be taken in two ways. We can either consider the accident as it is contained in this particular substance, or not as contained in a particular substance. If we consider it as in a particular, then as such it is not common to many, but considered apart from particularity, it can be common to many.
We can consider a substance in the same way. In other words, we can choose to consider a particular substance, apart from its particularity and as communicable to each, for example “man” or “dog”.
And so there is a forfold distinction is things; for all are either substance or accidents, and each of these can be taken either in its particularity, or in its universality. How we understand “what comes to be” must be an account of things within one or more of these categories.