The judgment of common sense as to any self-evident truth, may be defined as the immediate or instinctive response that is given (in psychological language) by the automatic action of the mind, or (in physiological language) by the reflex-action of the brain to any question which can be answered by such a direct appeal. The nature and value of that response will depend upon the acquired condition of the mind, or of the brain, at the time it is given; that condition being the general resultant of the whole psychical activity of the individual. The particular form of that activity is determined, as we have seen, in the first place, by his original constitution; secondly, by the influences which have been early brought to bear upon it from without; and thirdly, by his own power of self-direction. And it may be said that while the elementary form of common sense depends mainly upon the first of these factors, its ordinary form chiefly arises out of the first and second, and its special forms almost exclusively out of the third;--the response being given, in each case, by a nervous mechanism, in the organization of which the generalized results of the past experiences of consciousness (whether of the race or of the individual) have become embodied. The parallel between the cerebral action which furnishes the mechanism of thought now under consideration, and the action of the sensori-motor apparatus which furnishes the mechanism of sense and motion, is extremely close. We have seen that there are certain sense-perceptions, which, although not absolutely intuitive, very early come to possess--in every normally constituted human being--the immediateness and perfection of those corresponding perceptions which are intuitive in the lower animals; and that with these are associated certain respondent motions, which, though acquired by practice in the first instance, ultimately come to be performed as by a second nature. Certain of these motions, such as walking erect, are universally acquired; and thus obviously come to be the expressions of the original endowments of the mechanism, trained by an experience very similar in the uniformity of its character to that which educates the elementary form of common sense. For it must be clear to any one who compares the erect progression of a child who has just learned to walk, with that of a dancing dog or even of a chimpanzee, that while experience makes its acquirement possible in each case, only an organism which is at the same time structurally adapted for erect progression and possessed of a special co-ordinating faculty, can turn such experience to full account. The balancing the body in the erect position at starting, the maintenance of that balance by a new adjustment of the centre of gravity as the base of support is shifted from side to side and from behind forwards, and the alternate lifting and advance of the legs, involve the harmonious co-operation of almost all the muscles in the body.