(2.) Somewhat less familiar, but not unimportant, is a consideration which I may express by calling it the necessary instability of mind, its variability from moment to moment. Your mind, my mind, every mind, is continually sustaining modifications, disintegrations, reconstructions, by all that acts upon it, by all it comes to know. We are much in the habit, indeed, of speaking of experience, of different kinds of knowledge, as so much material for the mind--material delivered into it, outspread as it were on its floor, and which it, the lord and master, may survey, let lie there for occasion, and now and then select from and employ. True! but not the whole truth! The mind does not stand amid what it knows, as something distinct and untouched; the mind is actually composed at any one moment of all that it has learnt or felt up to that moment. Every new information received, every piece of knowledge gained, every joy enjoyed, every sorrow suffered, is then and there transmuted into mind, and becomes incorporate with the prior central substance. To resort now to that mechanical figure which I said might be found useful: every new piece of information, every fact that one comes to apprehend, every probability brought before one in the course of life, is not only so much new matter for the thinking apparatus to lay hold of and work into the warp and woof of thought; it is actually also a modification of the thinking apparatus itself. The mind thinks _with_ what it knows; and, if you alter the knowledge in any one whit, you alter the thinking instrumentality in proportion. Our whole practice of education is based on this idea, and yet somehow the idea is allowed to lurk. It may be brought out best perhaps by thinking what may happen to a mind that has passed the period of education in the ordinary sense. A person of mature age, let us say, betakes himself, for the first time, to the study of geology. He gains thereby so much new and important knowledge of a particular kind. Yes! but he does more. He modifies his previous mind; he introduces a difference into his mode of thinking by a positive addition to that instrumentality of notions _with_ which he thinks. The geological conceptions which he has acquired become an organic part of that reason, that intellect, which he applies to all things whatsoever; he will think and imagine thenceforward with the help of an added potency, and, consequently, never again precisely as he did before. Generalize this hint, and let it run through history. The mind of Man cannot remain the same through two consecutive generations, if only because the knowledge which feeds and makes mind, the notions that constitute the thinking power, are continually varying. In this age of a hundred sciences, all tramping on Nature's outside with their flags up, and marching her round and round, and searching her through and through for her secrets, and flinging into the public forum their heaps of