"We must therefore make the distinction and say, that whilst no primary agent or prime efficient produces effects beyond its powers, every instrumental agent may exceed its own proper powers in action; for it acts not merely by its own virtue but by the virtue of a superior efficient....
"Since the blood acts, then, with forces superior to the forces of the elements, and exerts its influence through these forces or virtues and is the instrument of the Great Workman, no one can ever sufficiently extol its admirable, its divine faculties.
"In the first place and especially, it is possessed by a soul which is not only vegetative, but sensitive and motive also. It penetrates everywhere and is ubiquitous; abstracted, the soul or the life too is gone, so that the blood does not seem to differ in any respect from the soul or the life itself (anima); at all events it is to be regarded as the substance whose act is the soul or the life. Such, I say, is the soul, which is neither wholly corporeal nor yet wholly incorporeal; which is derived in part from abroad and is partly produced at home; which in one way is part of the body, but in another is the beginning and cause of all that is contained in the animal body, viz., nutrition, sense, and motion, and consequently of life and death alike; for whatever is nourished, is itself vivified, and _vice versâ_. In like manner that which is abundantly nourished increases; what is not sufficiently supplied shrinks; what is perfectly nourished preserves its health; what is not perfectly nourished falls into disease. The blood therefore, even as the soul, is to be regarded as the cause and author of youth and old age, of sleep and waking, and also of respiration. All the more and especially as the first instrument in natural things contains the internal moving cause within itself. It therefore comes to the same thing, whether we say that the soul and the blood, or the blood with the soul, or the soul with the blood performs all the acts in the animal organism." A lame and impotent conclusion which does not advance our knowledge, though perhaps it was the most plausible that could be drawn from the premisses at Harvey's command. Indeed he was himself dissatisfied with his conception of the vital principle, for in another essay after a discussion to show that the egg is not the product of the body of the hen, but is a result of the vital principle, he turns away from the subject with evident relief to more profitable subjects, and with the words "Leaving points that are doubtful and disquisitions bearing upon the general question, we now approach more definite and obvious matters."