“Every state of consciousness has its physical correlate,” says Helmholtz,[68] and this proposition contains the all in all of our problem. Every mental event must have its corresponding physical event[69] in some form, and is therefore capable of being sensed, or known to be indicated by some trace. Identical inner states do not, of course, invariably have identical bodily concomitants, neither in all individuals alike, nor in the same individual at different times. Modern methods of generalization so invariably involve danger and incorrectness that one can not be too cautious in this matter. If generalization were permissible, psychical events would have to be at least as clear as physical processes, but that is not admissible for many reasons. First of all, physical concomitants are rarely direct and unmeditated expressions of a psychical instant (e.g., clenching a fist in threatening). Generally they stand in no causal relation, so that explanations drawn from physiological, anatomical, or even atavistic conditions are only approximate and hypothetical. In addition, accidental habits and inheritances exercise an influence which, although it does not alter the expression, has a moulding effect that in the course of time does finally so recast a very natural expression as to make it altogether unintelligible. The phenomena, moreover, are in most cases personal, so that each individual means a new study. Again the phenomena rarely remain constant; e.g.: we call a thing habit,--we say, “He has the habit of clutching his chin when he is embarrassed,”--but that such habits change is well known. Furthermore, purely physiological conditions operate in many directions, (such as blushing, trembling, laughter,[70] weeping, stuttering, etc.), and finally, very few men want to show their minds openly to their friends, so that they see no reason for co-ordinating their symbolic bodily expressions. Nevertheless, they do so, and not since yesterday, but for thousands of years. Hence definite expressions have been transmitted for generations and have at the same time been constantly modified, until to-day they are altogether unrecognizable. Characteristically, the desire to fool others has also its predetermined limitations, so that it often happens that simple and significant gestures contradict words when the latter are false. E. g., you hear somebody say, “She went down,” but see him point at the same time, not clearly, but visibly, _up_. Here the speech was false and the gesture true. The speaker had to turn all his attention on what he wanted to say so that the unwatched co-consciousness moved his hand in some degree.