A similar exclusiveness vitiates the materialist doctrine. Overlooking the primary fact that Feeling is indissolubly interwoven with processes regarded as purely physical because they are considered solely in their objective aspect, the materialist fails to recognize the operation of psychological laws in the determination of physiological results; he hopes to reduce Biology to a problem of Mechanics. But Vitality and Sensibility are coefficients which must render the mechanical problem insoluble, if only on the ground that mechanical principles have reference to quantitative relations, whereas vital relations are qualitative. His error is the obverse of the vitalist’s error. The vitalist imagines that the speciality of organic phenomena proves the existence of a cause which has no community with the forces operating elsewhere; so, turning his back on all the evidence, he attempts to explain organic phenomena without any aid from Physics and Chemistry. The materialist, turning his back on all the evidence of quite special conditions only found at work in living organisms, tries to explain the problem solely by the aid of Physics and Chemistry. It is quite certain that physiological and psychological problems are not to be solved if we disregard the laws of Evolution through Epigenesis. The mental structure is evolved, as the physical structure is evolved. It is quite certain that no such evolution is visible in anorganisms, nor will any one suppose it to be possible in machines. From the biological point of view we must therefore reject both Idealism and Materialism. We applaud the one when it says, “Don’t confuse mental facts by the introduction of physical hypotheses”; and the other when it says, “Don’t darken physical facts with metaphysical mists.” We say to both, “By all means make clear to yourselves which aspect of the phenomena you are dealing with, and express each in its own terms. But in endeavoring to understand a phenomenon you must take into account all its ascertainable conditions. Now these conditions are sometimes only approachable from the objective side; at other times only from the subjective side.”