The Samkhya school builds its entire account of reality on a single, uncompromising division: consciousness and matter are two genuinely separate and irreducible principles, and nothing in the system allows one to turn into or be explained by the other. Purusha, pure consciousness, does nothing at all; it only witnesses, silently and without acting, everything that unfolds before it. Prakriti, unconscious nature, does everything else, unfolding on its own, without any guiding will, into mind, ego, the senses and the physical elements the material world is made of, driven by the interplay of three qualities called gunas, lightness and clarity, activity and passion, and inertia and darkness. Ishvarakrishna's Samkhyakarika, composed in a spare seventy two verses around the fourth or fifth century, systematized this scheme into twenty five fundamental principles, or tattvas, purusha standing as the twenty fifth and final one, the silent witness for whose sake the other twenty four, all products of prakriti alone, are said to exist and unfold in the first place. What the manifest world actually is, on this account, is nothing more than prakriti being observed, a claim of two irreducible principles held together, one wholly passive and one wholly active, neither one ever collapsing into or explaining the other.