 There is one other interesting general feature of this scheme that is worth pointing out before looking at its details. Aristotle’s rejection of the view that being is a genus and his subsequent acceptance of ten distinct highest kinds leads to a doctrine concerning being itself that is at the center of Aristotle’s Metaphysics. (It should be noted, however, that there is genuine disagreement over the extent to which Aristotle accepted the doctrine of being that appears in the Metaphysics when he wrote the Categories.) According to Aristotle, some words do not express a genus but instead are what he calls pros hen homonyms — that is, homonyms related to one thing (pros hen), variously called cases of ‘focal meaning’ or ‘focal connection’ or ‘core-dependent homonymy’ in the literature on this topic (1003a35 ff.). Such words are applicable to various items in the world in virtue of the fact that those items all bear some type of relation to some one thing or type of thing. An example of such a homonym, according to Aristotle, is ‘healthy’. A regimen, he says, is healthy because it is productive of health; urine is healthy because it is indicative of health; and Socrates is healthy because he has health. In this case, a regimen, urine, and Socrates are all called ‘healthy’ not because they stand under some one genus, namely healthy things, but instead because they all bear some relation to health. Similarly, according to Aristotle, things in the world are not beings because they stand under some genus, being, but rather because they all stand in a relation to the primary being, which in the Categories he says is substance. This explains in part why he says in the Metaphysics that in order to study being one must study substance (1004a32, 1028a10–1028b8).