Here is a philosophical viewpoint that has been put forth by Roger Penrose about what exactly composes reality. In this philosophical viewpoint, Reality is only understood by the Mind when it has obtained an objective understanding of (1) the intricacies involved in the Platonic Mathematical World, (2) how the Platonic Mathematical World represents the Physical World that we live in, and (3) how the Physical World we live in gives rise to the Mind that has the ability to comprehend both the Platonic and the Physical Worlds.
Update 12/26/10 - The opening theme song to The Twilight Zone outlines this philosophy of 3 World Reality as well. "3 World Reality - a place where the Mind's Eye, the dimension of Inner Knowledge (The Mental World), and the Mind's Ear, the study of Outer Knowledge (Nature) meet, and where the mind charts a course through the Logos of the Unknown (Platonic Realm of Abstract Forms and Ideas)..."