For Plato, the only suitable instrument for knowledge of the real world is reason and understanding. He defines understanding as the highest activity of the soul and reason as the second-highest activity of the soul. (Republic, 511c) These activities are necessary to glimpse the things of the real word, the actual Forms contained in the world of Forms. (Republic, 509d). For Plato, true Knowledge was the Knowledge of these real things. (Republic, 509e). For him, all Knowledge was Knowledge of something that exists because what does not exist is nothing, of which it is impossible to have Knowledge. (Republic, 477e) Through the proposition that knowledge and opinion are different capacities, Plato infers that knowledge and opinion must be directed at different objects, though our daily experience tells us otherwise. Thus, Plato comes to the conclusion that no subject that we can have opinions on can be known. This is a very nonsensical conclusion and Kant's discussion of ideas and understanding helps...

Conventional opinion and knowledge is what Kant may call a pure form of understanding, a Category. However, Opinion and Knowledge in the sense that Plato uses it is only a pure form of reason, an Idea. This is Plato created new definitions and rules for Opinion and Knowledge, which did not comport with our existing understanding of them. Thus, the reason Plato argued that we could never see these Forms, e.g. Knowledge and Opinion, through sense-perception was that they cannot be experienced. The Theory of Ideas is difficult and even impossible to refute because they can never be experienced, partly because, having never experienced them, we would not be able to recognize them.
Bibliography

Kant, Immanuel. Prolegomena to Any Future Metaphysics. James Fieser. 1997. Internet resource

Plato, Benjamin Jowett, and Irwin Edman. The Works of Plato. New York: Modern Library, 1928. Print.