Experience instructs us that these mental forces manifest themselves by the intermediary of special organisms, very complex in the case of man and the superior animals, but simple in that of the lowest, among which latter class mind has no need of muscles and nerves in order to manifest itself externally, but seems to act directly upon a homogeneous matter, the movements of which it determines (the amoeba of Ehrenberg). It is in these elementary organizations that the problem of the action of mind on matter is stated, after a fashion, in its simplest terms. When once we have admitted the existence of the will as distinct, at least in principle, from the material body, it becomes solely a question of experience to ascertain whether other wills than that of man and the animals play any role whatever, frequent or occasional, on the stage of life. If these wills exist, they will have some means or other of manifestation, with which _experience alone_ can make us acquainted. As a matter of fact, all that it is possible to affirm, _a priori_, is that, in order to appear, they _must_ manifest themselves through some one of the forms of the eternal substance we call matter. But, to say that this matter must necessarily have an organization of muscles, nerves, etc., would be to hold to a very narrow idea, and one already belied by observation of the animal kingdom in its lower types. As long as we do not know what the bond is that unites the mind to the matter in which it manifests itself, it would be perfectly illogical to lay down, _a priori_, particular conditions which matter must observe in this manifestation. These conditions are at present wholly undetermined. Thus we are at liberty to seek for signs of these manifestations in the cosmic ether or in ponderable matter; in the gases, the liquids, or the solids; in unorganized matter, or particularly in matter already organized, such as that of which man and the animals are built up. It would be poor logic to affirm that other wills than those of men and animals cannot be discovered, on the ground that, heretofore, nothing of the kind has been seen; for facts of this kind may have been observed, but not scientifically elucidated and authenticated. Furthermore these wills might appear only at long intervals, or what seem long to us; but the vast abysses of nature's epochs are not to be spanned by our little memories or measured by the momentary duration of our lives.