In fact, all which is not actually present to the mind--and the present is an infinitesimal fraction of knowledge--is reproduced by the memory, and this is effected by the molecular movements of the human brain, and by what may be called the ethereal modifications which took place when the sensations, perceptions, and acts first occurred. If the cells vibrate, and the organs of the brain are affected by the recollection of past ideas and acts, just as when they actually occurred (and this appears from Schiff's experiences as to the increase of the brain in heat and volume during dreams), this vibration will be still more marked when any quality which affects our senses is reproduced in the mind. The particular _form_ of the quality as it appears in a definite object is certainly wanting in the abstract conception; it remains in the first stage of pure sensation, like a spontaneous act of observation, and it is transformed into apprehension by the mental faculty. But the inward consciousness of the quality is actual, psychical, and physical. The abstract conception is a psychical symbol composed of idea and consciousness, or rather of act and consciousness; both are fused into a logical conception of indefinite form, yet consisting of real elements, that is, of cerebral motions and of sensations. Estimated according to its genuine value, therefore, an abstract conception may be divided into three classes--physical, moral, and intellectual. Whiteness and colours in general, levity and weight, hardness, sound, and the like qualities, are all abstract types which belong to the physical class. Goodness, virtue, love, hatred, and anger must be assigned to the moral class; and equality, identity, number, and quantity, etc., to the intellectual class. Such abstract conceptions, without which human speech would be impossible, did not in the case of primitive man take the explicit and reflex form in which they are presented by mature science, and it is expedient to inquire what character they really assumed in the spontaneous exercise of thought and speech. There is certainly a difference between the mythical and specific types and the intrinsic value of these abstract conceptions. The former served for the causative interpretation of the living system of the world, and had a superstitious influence on the moral and social progress of mankind; the latter were merely the instrument of thought and speech, and were in spontaneous and daily use. But in spite of this difference, there was no radical and substantial diversity in the genesis of such conceptions, and the fundamental elements of perception were common to both. While the form varied, the primitive law and genesis remained the same.