He is talking about the difference between intension and extension. The properties "creature with a heart" and "creature with kidneys" are different, even though they may have the same extension (if the set of creatures with a heart and the set of creatures with a kidney happen to be the same). This also applies to relations of arbitrary arity. In mathematics everything is usually treated as extensional, because all the mathematical objects, like numbers, exist "necessarily". This is not the case for other objects, where things could be the same (like the set of creatures with heart and the set of creatures with kidneys) but they aren't necessarily the same. It's possible that there is a creature with heart but without kidneys. Though even in mathematics, properties that define the same objects are often not trivially equivalent: they are necessarily equivalent, but it may take a complex proof to show that they are.