Some things (if they exist at all) are present only “in” other things: a smile, a haircut (product, not process), a hole …. Such things may be opposed to things that exist “in their own right”. Metaphysicians call the things that exist in their own right ‘substances’. Aristotle called them ‘protai ousiai’ or “primary beings”. They make up the most important of his ontological categories. Several features define protai ousiai: they are subjects of predication that cannot themselves be predicated of things (they are not universals); things exist “in” them, but they do not exist “in” things (they are not accidents like Socrates' wisdom or his ironic smile); they have determinate identities (essences). This last feature could be put this way in contemporary terms: if the prote ousia x exists at a certain time and the prote ousia y exists at some other time, it makes sense to ask whether x and y are the same, are numerically identical (and the question must have a determinate answer); and the question whether a given prote ousia would exist in some set of counterfactual circumstances must likewise have an answer (at least if the circumstances are sufficiently determinate—if, for example, they constitute a possible world.  More on this in the next section). It is difficult to suppose that smiles or holes have this sort of determinate identity. To ask whether the smile Socrates smiled today is the smile he smiled yesterday (or is the smile he would have smiled if Crito had asked one of his charmingly naïve questions) can only be a question about descriptive identity.