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.