From the _form of time and the single dimension_ of the series of ideas, on account of which, in order to take up one, the intellect must let all the others fall, there follows not only its distraction, but also its _forgetfulness_. Most of what it lets fall it never takes up again; especially since the taking up again is bound to the principle of sufficient reason, and thus demands an occasion which the association of thoughts and motivation have first to supply; an occasion, however, which may be the more remote and smaller in proportion as our sensibility for it is heightened by our interest in the subject. But memory, as I have already shown in the essay on the principle of sufficient reason, is not a store-house, but merely a faculty acquired by practice of calling up ideas at pleasure, which must therefore constantly be kept in practice by use; for otherwise it will gradually be lost. Accordingly the knowledge even of the learned man exists only _virtualiter_ as an acquired facility in calling up certain ideas; _actualiter_, on the other hand, it also is confined to one idea, and is only conscious of this one at a time. Hence arises a strange contrast between what he knows _potentiâ_ and what he knows _actu_; that is, between his knowledge and what he thinks at any moment: the former is an immense and always somewhat chaotic mass, the latter is a single distinct thought. The relation resembles that between the innumerable stars of the heavens and the limited field of vision of the telescope; it appears in a striking manner when upon some occasion he wishes to call distinctly to his remembrance some particular circumstance in his knowledge, and time and trouble are required to produce it from that chaos. Rapidity in doing this is a special gift, but is very dependent upon day and hour; therefore memory sometimes refuses us its service, even in things which at another time it has readily at hand. This consideration calls us in our studies to strive more to attain to correct insight than to increase our learning, and to lay it to heart that the _quality_ of knowledge is more important than its _quantity_. The latter imparts to books only thickness, the former thoroughness and also style; for it is an _intensive_ quantity, while the other is merely _extensive_. It consists in the distinctness and completeness of the conceptions, together with the purity and accuracy of the knowledge of perception which forms their foundation; therefore the whole of knowledge in all its parts is penetrated by it, and in proportion as it is so is valuable or trifling. With a small quantity, but of good quality, one achieves more than with a very large quantity of bad quality.