 According to the traditional interpretation, here we have the claim that x and y are numerically identical (or “one in number”) if, and only if, they have the same matter (or the matter of x is “one” with the matter of y). An alternative reading takes this passage to be about unity rather than individuation: Aristotle would be saying that x is numerically one if and only if x’s matter is one, where a thing’s matter being “one” means that it is one continuous piece (of bone, for example).