How do we know the Forms, according to Plato?

In the Republic, Plato claims that the Forms initially become known to humans before birth. We spend our lives recollecting their natures, and we commune with them again after our deaths. Prior to being born, Plato holds that we spent eternity past learning the Forms and their natures through communing with them in their realm. The doctrine or recollection that he espouses holds that we then spend our lives remembering the natures of the...

In the Republic, Plato claims that the Forms initially become known to humans before birth. We spend our lives recollecting their natures, and we commune with them again after our deaths. Prior to being born, Plato holds that we spent eternity past learning the Forms and their natures through communing with them in their realm. The doctrine or recollection that he espouses holds that we then spend our lives remembering the natures of the Forms that we knew. This notion of innate knowledge claims that the things that we experience in the real world are shadows of the eternal Forms that they participate in, or copy. So we can look at two sticks of unequal length and realize that they are unequal, without having ever actually seen two sticks of exactly equal length. Although our experience of the world is imperfect, we understand that experience in the context of a latent knowledge of the Forms of universals such as Truth, Justice, and Equality. Even though we may never have seen true examples of these things (that is, in this world all examples of universals are defective in some respect), we have an understanding of what they are due to our communion with the Forms in our pre-life. Thus we are able to see faulty ethics, injustice and ugliness and recognize them for what they are. Plato also holds that after our deaths we will return to the realm of the Forms and again contemplate their natures.

The ancient Greek philosophers were more interested in these abstract universals I mentioned above than in the nature of artifacts, but sometimes in an effort to make a concept easier to understand a teacher will explain “nature” in terms of a man-made or physical object, such as a chair or a tree.

Important works by Plato that address the concept of recollection are the Republic and the Meno. I've included links to the study guides for these works.