In considering what we can know about God, Thomas Aquinas (c. 1224–1274) held that there were three basic sources of knowledge: natural reason (which is rooted in the observation of the effects of God's activities embodied in his creatures), public revelation (of which the Bible is the most prominent instance) and mystical experience (which, because of its private nature, cannot supply evidence to the science of theology). Grounding his reflections in the philosophy of Aristotle and developing a concept of analogical knowledge, Aquinas argued that natural reason could readily attain to the knowledge of God's existence as well as to a limited set of divine attributes (e.g., necessity, eternity, immateriality, perfection, goodness, simplicity, uniqueness, intelligence, etc). Public revelation, which on his view it is reasonable to accept, affirms what is knowable by natural reason and supplements it in very substantial ways with knowledge that natural reason cannot attain by itself.