This worship of animals was a grievous puzzle to the philosophers of the classical age. The venerable antiquity of Egypt, the high level of its moral code, and, above all, the spiritual and exalted character of so much of its religion, had deeply impressed the thinking world of the Roman Empire. That world had found, in a blending of Egyptian religious ideas with Greek metaphysics, a key to the mysteries of life and death; in the so-called Hermetic books the old beliefs and religious conceptions of Egypt were reduced to a system and interpreted from a Greek point of view, while the Neo-Platonic philosophy was an avowed attempt to combine the symbolism of Egypt with the subtleties of Greek thought. But the animal worship was hard to reconcile with philosophy; even symbolism failed to explain it away, or to satisfy the mind of the inquirer. Plutarch had boldly denied that the worship of an animal was in any way more absurd than that of an image; the deity, if so he chose, could manifest himself in either equally well. Porphyry had recourse to the doctrine of the transmigration of souls. If the soul migrated after death into the body of some lower animal, he urged, it would communicate to the latter a portion of the divine essence. But after all this was no explanation of the worship paid to the animal; the soul had not been worshipped while it was still in the body of its original possessor, and there was therefore no reason why it should be worshipped when it was embodied in another form. Moreover, metempsychosis in the Greek sense was never an Egyptian doctrine. All the Egyptian held was that the soul, after it had been justified and admitted to a state of blessedness, could enter for a time whatever material form it chose; could fly to heaven, for instance, in the body of a swallow, or return to the mummified body in which it had once dwelt. But such embodiments were merely temporary, and matters of free choice; they were like a garment, which the soul could put on and take off at will.