The genus and species concepts that we find in the categories are what mediaeval philosophers took to be universals, which following Aristotle they unanimously agreed are what is “predicable of many” (On Interpretation 7, 17a38–b1). No one disagreed that the term ‘humanity’ was predicable of many particular human beings. The pressing metaphysical question was rather whether there is a property of being-human, i.e. humanity, that all singular human beings share or not, which would explain why it is true to say that Plato and Socrates, for instance, are human beings. The realist holds that there is some kind of universal entity – a repeatable and instantiable property – that all the particular entities or members of some kind exemplify. But since it is obvious that Socrates and Plato are both human beings and yet that each is a singular, individual entity, the realist also has to give some account of how the property of being-human is individuated so as to constitute Socrates on the one hand and Plato on the other. The nominalist, by contrast, denies that any such universal entity exists, holding instead that all entities are just particular.