1. We have examined the proposed "ten genera": we have discussed also the theory which gathers the total of things into one genus and to this subordinates what may be thought of as its four species. The next step is, naturally, to expound our own views and to try to show the agreement of our conclusions with those of Plato.
Now if we were obliged to consider Being as a unity, the following questions would be unnecessary:
Is there one genus embracing everything, or are there genera which cannot be subsumed under such a unity? Are there first-principles? Are first-principles to be identified with genera, or genera with first-principles? Or is it perhaps rather the case that while not all genera are first-principles, all first-principles are at the same time genera? Or is the converse true? Or again, do both classes overlap, some principles being also genera, and some genera also principles? And do both the sets of categories we have been examining imply that only some principles are genera and some genera principles? or does one of them presuppose that all that belongs to the class of genera belongs also to the class of principles?
Since, however, we affirm that Being is not a unity- the reason for this affirmation is stated by Plato and others- these questions become imperative, once we are satisfied as to the number of genera to be posited and the grounds for our choice.
The subject of our enquiry, then, is the Existent or Existents, and it presents immediately two problems demanding separate analysis:
What do we mean by the Existent? This is naturally the first question to be examined.
What is that which, often taken for Being [for the Existent], is in our view Becoming and never really Being? Note however that these concepts are not to be taken as distinguished from each other in the sense of belonging to a genus, Something, divided into Being and Becoming; and we must not suppose that Plato took this view. It would be absurd to assign Being to the same genus as non-Being: this would be to make one genus of Socrates and his portrait. The division here [between what has Being and what is in Becoming] means a definite marking-off, a setting asunder, leading to the assertion that what takes the appearance of Being is not Being and implying that the nature of True Being has been quite misapprehended. Being, we are taught, must have the attribute of eternity, must be so constituted as never to belie its own nature.
This, then, is the Being of which we shall treat, and in our investigation we shall assume that it is not a unity: subsequently we ask leave to say something on the nature of Becoming and on what it is that comes to be, that is, on the nature of the world of Sense.