After briefly putting Porphyry’s On Abstinence from Animal Food into its historical context, I present two biological theories which appear in this treatise: the first may be called “providential ecology,” the theory that the natural world operates very well without the intervention of man, that God or Nature takes care of biological balance most effectively without human intervention; the second may be called “the rationality of animals,” the theory that there is no radical distinction between human reason and the rationality displayed by animals. Both theories may be placed into the general philosophical position elaborated by the earlier Neoplatonists conceming man’s place in nature, the character of embodied souls, and the interrelationships between being, God, and λόγοϛ; I have not, however, set myself this larger task.