(3.) _The Relation of Science to Thinking._—In opposition to the Protagorean sensualism, we have already referred to an energy of the soul independent of the sensuous perception and sensation, competent in itself to examine the universal, and grasp true being in thought. There is, therefore, a double source of knowledge, sensation and rational thinking. Sensation refers to that which is conceived in the constant becoming and perpetual change, to the pure momentary, which is in an incessant transition from the was, through the now, into the shall be (_Parm._ p. 152); it is, therefore, the source of dim, impure, and uncertain knowledge; thinking on the other hand refers to the abiding, which neither becomes nor departs, but remains ever the same. (_Tim._ p. 51.) Existence, says the Timæus (p. 27) is of two kinds, “that which ever is but has no becoming, and that which ever becomes but never is. The one kind, which is always in the same state, is comprehended through reflection by the reason, the other, which becomes and departs, but never properly is, may be apprehended by the sensuous perception without the reason.” True science, therefore, flows alone from that pure and thoroughly internal activity of the soul which is free from all corporeal qualities and every sensuous disturbance. (_Phæd._ p. 65.) In this state the soul looks upon things purely as they are (_Phæd._ p. 66) in their eternal being and their unchangeable condition. Hence the true state of the philosopher is announced in the Phædon (p. 64) to be a willingness to die, a longing to fly from the body, as from a hinderance to true knowledge, and become pure spirit. According to all this, science is the thinking of true being or of ideas; the means to discover and to know these ideas, or the organ for their apprehension is the dialectic, as the art of separating and combining conceptions; the true objects of dialectics are ideas.