Read: Republic, Books VI and VII, Selection of fragments from Parmenides, Selection from Plato’s dialogue Parmenides (latter two on Courseworks)
At the end of Book V (476), Socrates makes a distinction between the Philosopher and the Lover of Sight and Sound, where the former has knowledge but the latter only has opinion. Further, we get the interesting exchange at 476e, “[D]oes the person who knows know something or nothing? – He knows something – Something that is or something that is not? – Something that is, for how could something that is not be known?” You saw similar sorts of claims in the Parmenides fragments, which also suggest the one can only know things about objects that are (i.e. that exists).
A general, very abstract, question: do you know (vs. believe, imagine, etc) anything about an object that doesn’t exist (e.g. a unicorn, Santa Claus)? If you think you might, give a candidate example, what you know about it, and how you came to know that. Why do you think your psychological attitudes constitute knowledge? If you think you can never have knowledge about things that don’t exist, why do you think that? Do you have other attitudes? Why those attitudes instead of knowledge? Either way, what do you think separates the two cases of objects that are and objects that are not in the case of knowledge?