must be reached by each thinker through a like process of ascent, it came to be for later antiquity the very type of the hopelessly obscure, and men would say: "As incomprehensible as the Good of Plato." In his own mind, however, it constituted a new type of absolute certitude, in default of which no genuine knowledge was possible. The supreme conception was reached by an involved and uncertain process of thought, but when it was thus reached its truth was immediately manifest to reason. The mental act by which this takes place Plato represents by the analogy of sensuous perception. In contrast to such perception, however, it possessed a mediated immediacy. In a word, it was an intuition. This logical theory, which with modifications of greater or less import has persisted down to our own day, descended to modern times by three principal avenues, the teaching of Augustine, that of Aristotle, and that of Plato himself. Aristotle, who gave to the method of working up to first principles the name of induction (èπayoyń) appears to have thought that it led, not to a single highest conception, but to a variety of first principles peculiar to the various special sciences; but each when reached was intuitively certain. With Augustine the intuition of self-consciousness first gains the importance which it has had in modern thought.

It is thus entirely in the spirit of the ancient rationalism that Descartes divides the task of philosophy into two parts: first, a preliminary analysis, the object of which is to discover the necessary fundamental truths; and, secondly, the deduction from these of the system of the sciences. The so-called 'criterion of truth' which he professed to use in order to distinguish genuine from pretended intuitions, is peculiarly significant. The genuine are clear (that is to say, indubitably present to consciousness) and distinct (that is to say, unmistakable in content). In both epithets the analogy of sense-perception is evident; and in both alike the recognition of an absolute beginning is apparent,-a beginning which lies beyond proof and beyond external criticism. The evidence of the intuition is entirely in itself. Reflection can do no more than note that it is and what it is.

A most instructive example of Descartes's intuitions is that which stands first in his system and which he accepts as the type and standard of them all,-self-consciousness. I may doubt (says he) the existence of all the objects of my thoughts, feelings, and desires. I may question whether the world of nature which the senses reveal be not an illusion; whether the whole content of the deductive sciences be not vitiated by lapses of memory; whether all the joys and sorrows of life, all life's purposes and ideals, be not wholly vain. But-past and future aside-I cannot question the reality of my present experience as such. I cannot doubt that such and such ideas, emotions, and impulses are now within my mind. Indeed all that I know assuredly with regard to my mind-or rather, to speak strictly, myself— is just the fact that I have such an experience. So much is clear and distinct. I think, therefore I am (or, I, as a thinking being, exist), is not a deduction, nor is it in need of deductive support. It stands in its own strength. It would be true, though all else were false.

It has been observed, that so far as awareness of one's own mental states is concerned, the principle of immediate certitude is equally acknowledged by rationalists and by empiricists. Indeed, the very example of an intuition which we have just taken from Descartes turns out, when carefully examined, to be a modified form of the doctrine of Protagoras, set forth (not as he had done, as a lesson drawn from experience, but) as an intuition. When one looks to see what meaning Descartes attributes to the I, or myself, one discovers that it is simply that which is intuitively known as thinking. And, if one further asks what a thinking being is, he replies: "It is a thing that doubts, understands, [conceives,] affirms, denies, wills, refuses; that imagines also, and perceives." All these properties unite in his nature, as certainly as he exists-even though they should convey to him no truth beyond their inherence in, and inseparability from, himself. Suppose, for example, that the perceptions of sense are false. "Let it be so. At all events it is certain that I seem to see light, hear

a noise, and feel heat; this cannot be false; and this is what is properly called perceiving (sentire), which is nothing else than thinking."

All this may be otherwise expressed by saying that rationalism, as well as empiricism, acknowledges the absolute trustworthiness of introspection as a source of truth. The difference between the two schools on this score is due, first, to the improvement of the method of introspection by Berkeley; and, secondly, to a consequent great divergence of opinion as regards the actual contents of the mind, revealed by introspection. The ultimate appeal, however, is to the same supreme authority.

The improvement in method is nowhere more strikingly illustrated than in the criticism of Descartes with which Berkeley introduces his own theory of the visual perception of distance. Descartes had seen that the altering convergence of the two eyes plays a frequent part in such perception; and he promptly attributed this part to the angle formed by the lines joining the two eyes to the observed object. The greater the angle, the nearer the object; and thus the idea of the angle must be the basis for a judgment as to the distance. But this, Berkeley says, is pure fiction. No such process of judgment takes place; and the idea of the angle, upon which the judgment is supposed to be based, is almost never present to consciousness. The defectiveness of Descartes's procedure is that he allowed himself to speculate as to what must be in the mind in order to account for the possibility of the given phenomenon (of distance-vision), instead of basing his explanation upon such facts as were known from direct observation. Since Berkeley's statement of the case is very brief, and since it marks an epoch in the history of science, a few lines may be profitably quoted. "It is evident that no idea. which is not itself perceived can be to me the means of perceiving any other idea .... But those lines and angles by means whereof some men pretend to explain the perception of distance are themselves not at all perceived, nor are they in truth ever thought 1Meditations, II; italics ours.

of by those unskillful in optics. . . . Every one is himself the best judge of what he perceives and what not. In vain shall all the mathematicians in the world tell me that I perceive certain lines and angles which introduce into my mind the various ideas of distance, so long as I myself am conscious of no such thing."1 The true explanation he finds in the sensations set up by the muscular contraction involved in converging the two eyes. His language here is equally interesting. "It remains that we inquire what ideas or sensations there be that attend vision, unto which we may suppose the ideas of distance are connected, and by which they are introduced into the mind.-And, first, it is certain by experience, that when we look at a near object with both eyes, according as it approaches or recedes from us, we alter the disposition of our eyes, by lessening or widening the distance between the pupils. This disposition or turn of the eyes is attended with a sensation, which seems to me to be that which in this case brings the idea of greater or lesser distance into the mind. Not that there is any natural or necessary connection between the sensation we perceive by the turn of the eyes and greater or lesser distance. But because the mind has, by constant experience, found the different sensations corresponding to the different dispositions of the eyes to be attended each with a different degree of distance in the object—there has grown an habitual or customary connexion between those two sorts of ideas;" just as, for example, the sound of a word becomes associated with its meaning. Here we have what is at least a plausible theory, based upon a genuine introspection.

It is with the confidence born of this improved method of introspection, that Berkeley ventures to question the existence in the mind of a distinct class of abstract general ideas over and above particular ideas. Here his polemic is directed against Locke; but at the same time it attacks the very foundations of (intuitionalistic) rationalism. For the essential mark of an intuition, and that which distinguishes it from an impression of the 1An Essay Towards a New Theory of Vision, §§ 10, 12.

2 Ibid., §§ 16, 17.

senses, it is absolute universality. The rationalists would not even admit that a genuinely universal idea could in any way be built up from the data of sensation. Generalized images (supposed to be formed by the blurring together of a great number of similar sense-perceptions) were, indeed, acknowledged to exist; but these were to be carefully distinguished from the true ideas of reason. Thus, for example, the somewhat dim and hazy image of a triangle that may start up in the mind at the mention of the word, was believed to be a radically distinct and separate thing from the scientific conception of triangle which is treated of in geometry. This whole distinction Berkeley proposed, not to demolish, but utterly to transform, by pointing out that universality of meaning is not primarily a peculiarity of origin or structure of ideas, but a peculiar function which certain ideas have acquired. That is to say,-"an idea, which, considered in itself, is particular, becomes general by being made to represent or to stand for all other particular ideas of the same sort." Now what evidence had he to support this position? Simply the fact that after careful introspection he could discover no such general ideas as Locke or the rationalists had described. "If any man has the faculty of framing in his mind such an idea of a triangle as is here [by Locke] described, it is in vain to pretend to dispute him out of it, nor would I go about it. All I desire is that the reader would fully and certainly inform himself whether he has such an idea or no. And this, methinks, can be no hard task for anyone to perform . . . . So long as I confine my thoughts to my own ideas divested of words, I do not see how I can easily be mistaken. The objects I consider, I clearly and adequately know. I cannot be deceived in thinking I have an idea which I have not. It is not possible for me to imagine that any of my own ideas are alike or unlike that are not truly so." The final appeal is thus to the same authority which Descartes too recognized as supreme and infallible, the immediate consciousness of the contents of one's own mind.

1Principles of Human Knowledge, Introduction, § 13; our italics