The substance of these doctrines has long ago been expressed by Newton and Kant, but through the connection and the clearness of this exposition I believe I have made it more intelligible, so that that insight is possible for all which I regarded as necessary for the justification of proposition No. 18. Second Half. The Doctrine of the Abstract Idea, or Thinking. Chapter V.(16) On The Irrational Intellect. It must be possible to arrive at a complete knowledge of the consciousness of the brutes, for we can construct it by abstracting certain properties of our own consciousness. On the other hand, there enters into the consciousness of the brute instinct, which is much more developed in all of them than in man, and in some of them extends to what we call mechanical instinct. The brutes have understanding without having reason, and therefore they have knowledge of perception but no abstract knowledge. They apprehend correctly, and also grasp the immediate causal connection, in the case of the higher species even through several links of its chain, but they do not, properly speaking, _think_. For they lack _conceptions_, that is, abstract ideas. The first consequence of this, however, is the want of a proper memory, which applies even to the most sagacious of the brutes, and it is just this which constitutes the principal difference between their consciousness and that of men. Perfect intelligence depends upon the distinct consciousness of the past and of the eventual future, _as such_, and in connection with the present. The special memory which this demands is therefore an orderly, connected, and thinking retrospective recollection. This, however, is only possible by means of _general conceptions_, the assistance of which is required by what is entirely individual, in order that it may be recalled in its order and connection. For the boundless multitude of things and events of the same and similar kinds, in the course of our life, does not admit directly of a perceptible and individual recollection of each particular, for which neither the powers of the most comprehensive memory nor our time would be sufficient. Therefore all this can only be preserved by subsuming it under general conceptions, and the consequent reference to relatively few principles, by means of which we then have always at command an orderly and adequate survey of our past. We can only present to ourselves in perception particular scenes of the past, but the time that has passed since then and its content we are conscious of only in the abstract by means of conceptions of things and numbers which now represent days and years, together with their content. The memory of the brutes, on the contrary, like their whole intellect, is confined to what they _perceive_, and primarily consists merely in the fact that a recurring impression presents itself as having already been experienced, for the present perception revivifies the traces of an earlier one.