logical psychology is the theory of the muscular sense. Destutt de Tracy maintained that without motion we could not have knowledge of the existence of bodies; for it is arrested movement that gives the sensation of resistance. The essential point in the theory is to distinguish the sense of effort from purely passive muscular sensations. If consciousness is a good judge in these matters, says Alexander Bain, we may say that in voluntary effort we have the feeling of a faculty experienced from within outward and not that of a sensible surface stimulated by an external agent and transmitting an impression from without to within the nervous centers. The sense of effort would then seem to be the feeling of the production of motion rather than of motion produced. It is anterior' and not posterior to the motion. Without going into detail, we can, according to Bain, refer all muscular sensations to two great classes: the sensation of tension, and that of motion. Tension is an act of effort in so far as it meets an invincible resistance, for example when it endeavors to raise a weight that is beyond its strength, or to stop a galloping horse. We can distinguish three distinct sensations in that of tension: pressure, traction, and weight. The first occurs when we wish to crush an object, as a nut, with the hands; the second, when we wish to lead an object, as a horse, or a man who is resisting us; and the third, when we lift a weight. The first is an effort of ourselves on the exterior object; the second, of the exterior object on us; and the third, an upward effort. The feeling of tension is the same, whether the extensor or flexor muscles are involved. It is in a certain manner the feeling of force in equilibrium with the exterior force, but at its limit, and unable to go farther.
It is surprising that Bain, in discussing what he called the sensation of motion, did not first ask if such a sensation exists. Without doubt, since we effect motion, there must be something in the consciousness that corresponds to it; but does that something resemble what we call a motion—that is, a displacement in space? We see that the question of the sensation of motion is closely bound with the idea of the perception of space, or with the most obscure and complex question of metaphysics. Without this notion of space, the muscular sensation could not even take the name of tension or of contraction, for these terms imply motion, and motion implies space. The only peculiar characteristic of muscular sensation appears to be fatigue. Effort is an internal fatigue distinct from the external fatigue which is imposed by causes foreign to us. It consists in giving one's self a fatigue by the production of a desired act. We thus see how apparently the most elementary questions are complicated with those of the highest order. What, for example, is a desired act? The study of the simplest sensation, therefore, involves a theory of the will.
