Thomas Aquinas on the Proportionate Causes of Living Species

The principle of proportionate causality is often cited as a cause for concern that Thomistic metaphysics may be irreconcilable with a theory of biological evolution. St. Thomas does hold that for the generation of what he calls perfect animals, a generator of the same species is required. This study clarifies what the proportionate causes of generated organisms are for Thomas, examining his views about spontaneous generation, reproductive generation, and hybridization, while also articulating the roles of both the heavenly bodies and their separate movers as universal causes of generation. This study establishes that Thomas’s assertion of the need for a univocal generator for perfect animals is grounded not in the principle of proportionate causality, but rather in physical and biological doctrines received from Aristotle and in a causal principle that seems reconcilable with biological evolution, namely, that a remote universal cause requires more mediating causes to produce more powerful effects.