vatism as an abstract principle, its exemplification in the history of the Church of Rome, from its encounter with Galileo onwards, is not encouraging. It is well to rest under the shadow of the authority of Aristotle and the leading school men; but we must remember that in questions which lie on the borderland between physics and metaphysics it was difficult to give a decision at a time when the development of the two sciences was so disproportionate. And the problem of vitality, about which philosophical tradition and the scientific revolution came into conflict, belongs to that neutral territory. The duty of the biologist is to extend his explanations as far as they are capable, and the metaphysician may discuss the residuum, if there be any.
It was held formerly, and is still held by many Catholic philosophers who adhere to the orthodox practice of disregarding contemporary activity, that the immaterial world first reveals its presence in plants. There is, unfortunately, much confusion in defining the terms “immaterial,” “spiritual,” etc.; but for my purpose it is sufficient to say that they held (and hold) that the phenomena of plant-life (growth, nutrition, etc.) cannot be explained by the properties of the matter of which the plant is composed; that, consequently, they reveal the working of some principle in the plant which is not matter. From this point Immaterialists began to beat a retreat early in this century; curious survivals of it are still met with—e.g., in Lepidi, a modern Catholic philosopher of some reputation. It would be idle to discuss the controversy; but the moral of the retreat is a serious one.
Plants were credited with an immaterial principle—a “soul,” as they did not hesitate to call it—because their properties were very different from those of ordinary matter; the possibility of material forces producing widely different results when they enter into certain highly complex combinations came to be recognized as vegetable physiology, and chemistry progressed. The principle of the argument was unsound; ammonia has properties remarkably different from those of hydrogen and nitrogen, yet some will say (many Catholic philosophers will say it) that a new principle must be introduced to explain the new properties, when hydrogen and nitrogen unite and ammonia is formed. So also from the different properties of the plant, its vital activities, it was evidently illogical to demand the admission of an immaterial source for them.
Now, the difficulty is that the argument for an immaterial principle in the “lower” animals is precisely of the same character. The vital forces in animals are very different from ordinary material forces; hence it is inferred that they are not material forces—they are the manifestation of an immaterial principle. But mere difference of properties does not suffice, as is proved from the earlier controversy; what degree of difference is necessary before we are logically justified in introducing a
