This sounds like a tautology, but it isn't : Some concepts can signify whole individual things. But there are concepts that signify only some aspect of the thing. If we now want to determine the extension of such a concept, we collect all existent entities that are, or contain, that which is signified by this concept. And then we say, that although this concept does not signify individual things as individual things, it nevertheless stands for these individuals. We have here to do with a difference between signification and supposition . The mentioned individuals are then the supposits of the particular aspect that is signified by the concept.