The Fifth of the Five Ways

[Belacqua]

Boru was correct. In Scholaticism, an "accident" is a unessential feature, kind of like the cars do not have to be any particular color.

When did Boru say that?

I don't think it was Boru who said that. It looks as though it was Grandizer.

Essential qualities are those which must be present for a thing to be what it is. Accidental qualities are things that may or may not be present, but don't change what the thing is.

So if we consider the case of a particular human being, for example Yo Yo Ma, the ability to play the cello is, in the Aristotelian sense, accidental. Obviously it's not an accident, in the modern English sense of the term, because Yo Yo Ma worked hard to learn it. But he would still be human with or without the ability. 

The ability to play the cello, though it's not acquired accidentally, is not an essential quality of being human. Therefore in Aristotelian terms, it is an accidental quality.

Likewise if someone has a tan, that is an accidental quality. Because when he gets pale again he's still human.

I often use T-shirts as an example of per accidens and per se. It's pretty easy to define what qualities must be present for a thing to be a T-shirt: a hole for your head, two sleeves, made of wearable material, etc. The accidental qualities are the things that may vary, while the thing is still a T-shirt: the color, the name of the band that's on the front, etc.