Here is an excerpt from Plato Book V:

SOCRATES: “Tell us this: does someone who knows know something or nothing?” You answer for him.

GLAUCON: I will answer that he knows something.

SOCRATES: Something that is or something that is not?

GLAUCON: That is. How could something that is not be known?

SOCRATES: We are adequately assured of this, then, and would remain so, no matter how many ways we examined it: what completely is, is com- pletely an object of knowledge; and what in no way is, is not an object of knowledge at all?

GLAUCON: Most adequately.

SOCRATES: Good. In that case, then, if anything is such as to be and also not to be, wouldn’t it lie in between what purely is and what in no way is?

I understand that the concept of “is” is related to something that exists or is true. But what does “being” mean in this context?

There is also this statement later on:

SOCRATES (in the context of belief): Do you know what to do with them, then, or anywhere bet- ter to put them than in between being and not being? Surely they cannot be more opaque than what is not, by not-being more than it; nor clearer than what is, by being more than it.

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    "Being" means "what is", but in Plato's system in addition to being (what is) and non-being (what is not) there is also something in between, the becoming, and the whole sensible world is in that category. So what you normally think exists - doesn't. Being is only reserved for the eternal forms. They alone can be grasped by reason and be objects of knowledge, and after them fleeting sensibles, grasped only by opinion and reasonless perception, are molded. This is further expounded on in Timeaus, see SEP, Being and Becoming.
    – Conifold
    Sep 8, 2020 at 5:52

1 Answer 1


Very useful: Charles Kahn, Some Philosophical Uses of ‘To Be’ in Plato (1981), reprinted into Essays on Being (Oxford University Press, 2009).

According to the author, Plato uses the verb einai (and its nominal forms on and ousia) in a philosophically loaded way, in connection with the notion of truth.

See page 105-106:

  1. Having knowledge entails grasping truth.
  1. Grasping truth entails grasping how things are (‘attaining ousia’ objectively understood).
  1. Grasping how things are entails judging that things are so (‘attaining ousia’ intentionally understood).
  1. Judging that things are so entails a whole range of concepts, including existence (being something, as a subject of attributes), predication (being x, having an attribute), and truth claim (putting subject and attribute together in being-so, as a mirror of the world).

Since the ousia of step 4 can be found ‘not in bodily sense experiences but in our reasoning concerning these’, it is in the latter and not in the former that knowledge itself must be located (Parm, 186d2–5).

For the context of the discussion about being/non-being, Parmenides is relevant. And see also Plato’s Parmenides.

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