Plato was one of the earliest philosophers to provide a detailed discussion of ideas (it must be noted that in Plato's Greek the word idea carries a rather different sense from our modern English term). Plato argued in dialogues such as the Phadeo, Symposium, Republic, and Timaeus that there is a realm of ideas or forms (eidei), which exist independently of anyone who may have thoughts of these ideas, and it is the ideas which distinguish mere opinion from knowledge, for unlike material things which are transient and liable to contrary properties, ideas are unchanging and nothing but just what they are. Consequently, Plato seems to assert that material things can only be the objects of opinion; real knowledge can only be had of unchanging ideas. Furthermore, ideas for Plato appear to serve as universals; consider the following passage from the Republic:
"We both assert that there are," I said, "and distinguish in speech, many fair things, many good things, and so on for each kind of thing."
"Yes, so we do."
"And we also assert that there is a fair itself, a good itself, and so on for all things that we set down as many. Now, again, we refer to them as one idea of each as though the idea were one; and we address it as that which really is."
"That's so."
"And, moreover, we say that the former are seen, but not intellected, while the ideas are intellected but not seen."
— Plato, Bk. VI 507b-c