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as an external stimulus, whose action first of all causes an image to arise in the brain, through the medium of which the will carries out the effect proper, an outward action of the body. Now, in the human species however, the place of such an image as this may be taken by a concept drawn off from former images of this kind by dropping their differences, which concept consequently is no longer perceptible, but merely denoted and fixed by words. As the action of motives accordingly does not depend upon contact, they can try their power on the will against each other: in other words, they permit a certain choice which, in animals, is limited to the narrow sphere of that which has perceptible existence for them; whereas, in man, its range comprises the vast extent of all that is thinkable: that is, of his concepts. Accordingly we designate as voluntary those movements which are occasioned, not by causes in the narrowest sense of the word, as in inorganic bodies, nor even by mere stimuli, as in plants, but by motives. 1 These motives however presuppose an intellect as their mediator, through which causality here acts, without prejudice to its entire necessity in all other respects. Physiologically, the difference between stimulus and motive admits also of the following definition. The stimulus provokes immediate reaction, which proceeds from the very part on which the stimulus has acted; whereas the motive is a stimulus that has to go a roundabout way through the brain, where its action first causes an image to arise, which then, but not till then, provokes the consequent reaction, which is now called an act of volition, and voluntary. The distinction between voluntary and involuntary movement does not therefore concern what is essential and primary,
1 I have shown the difference between cause in its narrowest sense, stimulus, and motive, at length in my The Two Fundamental Problems of Ethics. [Essay on the Freedom of the Will, pp. 29–33.]
