Mind, like each of the physical forces may be {97} viewed as a mode of energy; it is essentially dynamic; activity or change, within or without us, appears to be the original source of all our mental impressions, and the cause of their re-excitement in an act of memory. A man's mind, being continually excited by circumstances, must be active whether he will or no, and if it does not possess sufficient truthful ideas entirely to occupy it, it must be more or less occupied with erroneous ones. "We can neither feel, nor know, without a transition or change of state--and every cognition, must be viewed as in relation to some other feeling, or cognition," (Bain. Mental and Moral Science, p. 83); _i.e._ the mental effect of impressions upon us depends upon our immediately previous mental state; consciousness and perception appear to be based upon cerebral change or activity; after strong excitement of consciousness an increased amount of acid products is found in the secretions. "It is a general law of the mental constitution, more or less recognised by inquirers into the human mind, that change of impression is essential to consciousness in every form," (Bain. Emotions and Will, 3rd edi. p. 550). A sufficient degree also of such change is a necessary condition of conscious perception; it is the stronger or more rapid only of mental changes that excite our consciousness. We perceive nearly all things by means of a difference of impression which they make upon us; by contrast. That which makes no such difference of impression, such as the great uniformities of time and {98} space, makes no immediate impression upon us. We only know of the existence of those uniformities by inference from our perceptions of sequences or of relative difference. Although the Earth moves at the rate of 62,000 miles an hour in its orbit, consciousness does not perceive it. If also there was no error, we should be less immediately able to discern truth, without pain we should lose much of the enjoyment of pleasure. Without the contrast of imperfection we could not directly appreciate perfection.