Platonism: One of the philosophical tenants Idealism is based on. In Borges' fiction, you find its manifestations in his questioning of reality. To illustrate this connection it's useful to compare it to Plato's "Allegory of the Caves" from Book VII The Republic. In this story Plato describes a group of people chained since birth in a cave. They are constrained in movement so they can only see themselves, objects, and others as shadows cast upon a screen. To them, the shadows are "real," because they cannot compare it with anything else. Yet when one of them is released, educated, and returns with the knowledge that shadows are not real, those that have remained deride him and argue that his "eyesight is spoiled" (187). Plato seems to suggest here that reality isn't an extrinsic, constant entity, but a personal mental construction, and as such, is mutable and variable instead of unchaning and static.

Plato. From The Republic. Classics of Western Philosophy. Ed. Steven M. Cahn.