James Adam, The Republic of Plato

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474C - 480A The philosopher, as analogy proves, is one who loves not a part of knowledge, but the whole. His passion is for Truth, and Truth means the Ideas. The Ideas are each of them One, but they appear many by union with particular things and one another. Lovers of sights and sounds and such like persons believe only in the many beautifuls; they cannot understand the One. Like dreamers, they mistake the copy for the original. Their condition of mind may be described as Opinion, that of the philosophers as Knowledge.

Let us proceed to prove this statement. The object of Knowledge ‘is’; that of Ignorance ‘is not.’ If therefore anything both ‘is’ and ‘is not,’ it must lie between Being and not-Being, and the faculty which cognizes it will be something between Knowledge and Ignorance.

‘Powers’ differ from one another according to the objects over which they preside, and the effects which they produce. The ‘power’ called Knowledge presides over Being, and produces the act of knowing. It is therefore different from the ‘power’ called Opinion, whose result is opining. What then is the object over which Opinion presides? We have seen that it is not Being; neither is it not-Being. Therefore Opinion is different both from Knowledge and from Ignorance. It is, in fact, something between Knowledge and Ignorance, less luminous than the one, more luminous than the other. Its object will therefore be that which both ‘is’ and ‘is not.’

Now it is just the many beautifuls etc. which both are and are not. There is not one of them which ‘is’ more than it ‘is not’ that which we say it is. We are therefore justified in saying that the many beautifuls etc. lie between Being and notBeing. Thus we have discovered the object of Opinion.

We conclude that those who have eyes for the many beautifuls etc., opine; while those who see the Beautiful itself, know. The former are lovers of Opinion, the latter lovers of Knowledge or philosophers.