When treating the question, How we know? Plato seems to understand by ideas not the ideas in the divine mind, but their copies impressed on matter, as the seal on wax. According to him, all knowing is by similitude, and as the idea leaves its exact image or form on matter, so by studying that image or copy we arrive at an exact knowledge of the idea or archetype in the divine mind. This is plain enough; but who are _we_ who study and know? Are we the archetypal idea, or are we its image or copy impressed on matter? Here is the difficulty we find in understanding Plato's doctrine of ideas. According to him all reality is in the idea, and what is not idea is phenomenal, unsubstantial, variable, and evanescent. The impress or copy on matter is not the idea itself, and is no more the thing itself than the reflection I see in a mirror is myself. Plato speaks of the soul as imprisoned in matter, and ascribes all evil to the intractableness of matter. Hence he originates or justifies that false asceticism which treats matter as impure or unclean, and makes the proper discipline of the soul consist in despising and maltreating the body, and in seeking deliverance from it, as if our bodies were not destined to rise again, and, reunited to the soul, to live forever. The real source of Manichaeism is in the Platonic philosophy. We confess that we are not able to make out from Plato a complete, coherent, and self-consistent doctrine of ideas. St. Thomas corrects Plato, and makes ideas the archetypes, exemplars, or models in the divine mind, and identical with the essence of God, after which God creates or may create existences. He holds the idea, as idea, to be _causa exemplaris_, not _causa essentialis_, and thus escapes both pantheism and dualism, and all tendency to either. {149} Aristotle, a much more systematic genius, and, in my judgment, a much profounder philosopher than Plato, rejects Plato's doctrine of ideas, and substitutes for them substantial forms, which in his philosophy mean real existences distinct from God; and which are not merely phenomenal, like Plato's copies on wax. True, he, as Plato, recognizes an eternal matter, and makes all existences consist of matter and form. But the matter is purely passive; and, as nothing, according to his philosophy, exists, save in so far as active, it is really nothing, exists only _in potentia ad formam_, and can only mean the ability of God to place existences after the models eternal in his own mind. His philosophy is, at any rate, more easily reconciled with Christian theology than is Plato's.