The beginner and founder of modern philosophy is Descartes. While he, like the men of the transition epoch just noticed, broke loose entirely from the previous philosophizing, and began his work wholly de novo, yet he did not content himself, like Bacon, with merely bringing out a new method, or like Boehme and his contemporaries among the Italians, with affirming philosophical views without a methodical ground. He went further than any of these, and making his standpoint one of universal doubt, he affirms a new, positive, and pregnant philosophical principle, from which he attempted logically to deduce the chief points of his system. The character and novelty of his principle makes him the beginner, and its inner fruitfulness the founder, of modern philosophy.

Rene Descartes (Renatus Cartesius) was born in 1596, at La Haye in Torraine. Possessing an independent property, he volunteered as a soldier in his twenty-first year, and served in the wars with the Dutch, the Bavarians, and the Imperialists. After this he travelled a good deal, and then abode a considerable time in Paris. In 1629 he left his native land, and betook himself to Holland, that he might there, undisturbed and unknown, devote himself to philosophy, and elaborate his scientific ideas. He spent twenty years in Holland, enduring much vexatious treatment from fanatical theologians, till in 1649 he accepted an invitation from Queen Christina of Sweden, to visit Stockholm, where he died in the following year.

The chief content of the Cartesian system may be seen condensed in the following epitome.

1. If science would have any thing fixed and abiding, it must begin with the primal ground of things; every presupposition which we may have cherished from infancy must be abandoned; in a word, we must doubt at every point to which the least uncertainty is attached. We must therefore doubt not only the existence of the objects of sense, since the senses so frequently deceive, but also the truths of mathematics and geometry—for, however evident the proposition may appear that two and three make five, or that the square has four sides, yet we cannot know but what God may have designedly formed us for erroneous judgments. It is therefore advisable to doubt every thing, in fact to deny every thing, to posit every thing as false.

2. But though we posit every thing as false to which the slightest doubt may be attached, yet we cannot deny one thing, viz., the truth that we, who so think, do exist. But rather from the very fact that I posit every thing as false, that I doubt every thing, is it manifest that I, the doubter, exist. Hence the proposition: I think, therefore I am (cogito ergo sum), is the first and most certain position which offers itself to every one attempting to philosophize. Upon this the most certain of all propositions, the certainty of all other knowledge depends. The objection of Gassendi that the truth of existence follows from any other activity of man as well as from thinking, that I might just as well say: I go to walk, therefore I exist,—has no weight; for, of all my actions, I can be absolutely certain only of my thinking.

3. From the proposition I think, therefore, I am, the whole nature of the mind may be determined. When we examine who we are who hold every thing to be false that is distinct from ourselves, we see clearly that neither extension nor figure, nor any thing which can be predicated of body, but only thought, belongs to our nature. I am therefore only a thinking being, i. e. mind, soul, intelligence, reason. Thought is my substance. Mind can therefore be apprehended clearly and completely for itself alone, without any of those attributes which belong to body. Its conception contains nothing of that which belongs to the conception of body. It is therefore impossible to apprehend it through any sensuous representation, or to make an image of it: it apprehends itself only through the pure intelligence.

4. From the proposition cogito ergo sum, follows still farther the universal rule of all certainty. I am certain that I am a thinking being, what now is involved in the fact that I am certain of any thing? Whence comes this certainty? From no other source than the knowledge that this first proposition contains a clear conception of that which I affirm. I know of a certainty that I am, and I know any thing else only when I know it as certainly as I know that I am. Hence I may regard it as a universal rule, that every thing is true which I know clearly and determinately.

5. This rule, however, is only a principle of certainty, not of knowledge and of truth. We apply it therefore to our thoughts or ideas, in order to discover what is objectively true. But our ideas are partly innate, partly acquired, and partly self-originated. Among those of the first class we find the idea of a God. The question arises, whence have we this idea? Manifestly not from ourselves; this idea could only be implanted within us by a being who has the fulness of all perfection in himself, i. e. only by an actually existing God. If I ask now the question, whence have I the faculty to conceive of a nature more perfect than my own? the answer must ever come, that I have it only from him whose nature is actually more perfect. All the attributes of God, the more I contemplate them, show that their idea could not have originated with myself alone. For though there might be in me the idea of substance because I am a substance, yet I could not of myself have the idea of an infinite substance, since I am finite; such an idea could only be given me through a substance actually infinite. Moreover, we must not think that the conception of the infinite is to be gained through abstraction and negation, as we might gain darkness through the negation of light; but I perceive, rather, that the infinite contains more reality than the finite, and that, therefore, the conception of the infinite must be correspondingly antecedent in me to that of the finite. Since then I have a clear and determined idea of the infinite substance, and since this has a greater objective reality than every other, so is there no other which I have so little reason to doubt. But now since I am certain that the idea of God has come to me from God himself, it only remains for me to examine the way in which I have received it from God. I have never derived it directly nor indirectly from the sense, for ideas through the sense arise only by affecting the external organs of sense; neither have I devised it, for I can neither add to it nor diminish it in any respect,—it must, therefore, be innate as the idea of myself is innate. Hence the first proof we can assign for the being of a God is the fact that we find the idea of a God within us, and that we must have a cause for its being. Again, the being of a God may be concluded from my own imperfection, and especially from the knowledge of my imperfection. For since I know that there is a perfection which is wanting in me, it follows that there must exist a being who is more perfect than I, on whom I depend and from whom I receive all I possess.—But the best and most evident proof for the being of a God is, in fine, that which is gained from the conception of a God. The mind among all its different ideas singles out the chiefest of all, that of the most perfect being, and perceives that this has not only the possibility of existence, i. e. accidental existence like all other ideas, but that it possesses necessary existence in itself. And as the mind knows that in every triangle its three angles are equal to two right angles, because this is involved in the very idea of a triangle, so does the mind necessarily infer that necessary existence belongs to the conception of the most perfect being, and that, therefore, the most perfect being actually exists. No other idea which the mind finds within itself contains necessary existence, but from the idea of the highest being existence cannot be separated without contradiction. It is only our prejudices which keep us from seeing this. Since we are accustomed in every thing to separate its conception from its existence, and since we often make ideas arbitrarily, it readily happens, that when we contemplate the highest being we are in doubt whether its idea may not be one also arbitrarily devised, or at least one in whose conception existence does not lie.—This proof is essentially different from that of Thomas (Anselm of Canterbury). His argument was as follows: “If we understand what is indicated by the word God, it is all that can be conceived of greatness; but now there is actually and in thought more belonging to him than the word represents, and therefore God exists not only in word (or representation), but in fact.” Here the defect in the syllogism is manifest, for from the premise it could only be concluded that God must therefore be represented as existing in fact, while his actual existence would not follow. My proof on the other hand is this,—we may predicate of a thing what we clearly see belongs to its true and changeless nature, or to its essence, or to its form. But now after we had examined what God is, we found existence to belong to his true and changeless nature, and therefore may we properly predicate existence of God. Necessary existence is contained in the idea of the most perfect being, not by a fiction of our understanding but because existence belongs to his eternal and changeless nature.

6. The result just found—the existence of God—is of the highest consequence. Before attaining this we were obliged to doubt every thing, and give up even every certainty, for we did not know but that it belonged to the nature of the human mind to err, but that God had created us for error. But so soon as we look at the necessary attributes of God in the innate idea of him, so soon as we know that he is true, it would be a contradiction to suppose that he would deceive us, or that he could have made us to err; for though an ability to deceive might prove his skill, a willingness to deceive would only demonstrate his frailty. Our reason, therefore, can never apprehend an object which would not be true so far as the reason apprehended it, i. e. so far as it is clearly known. For God might justly be styled a deceiver if he had given us a reason so perverted as to hold the false for the true. And thus every absolute doubt with which we began is dispelled. From the being of God we derive every certainty. For every sure knowledge it is only necessary that we have clearly known a thing, and are also certain of the existence of a God, who would not deceive.

7. From the true idea of God follow the principles of a philosophy of nature or the doctrine of the two substances. Substance is that which so exists that it needs nothing else for its existence. In this (highest) sense God is the only substance. God, as the infinite substance, has his ground in himself, is the cause of himself. The two created substances, on the other hand, the thinking and the corporeal substance, mind and matter, are substances only in a broader sense of the word; they may be apprehended under the common conception that they are things which need only the co-operation of God for their existence. Each of these two substances has an attribute which constitutes its nature and its essence, and to which all its other determinations may be referred. The attribute and essence of matter is extension, that of mind, thought. For every thing else which can be predicated of body presupposes extension, and is only a mode of extension, as every thing we can find in mind is only a modification of thought. A substance to which thought immediately belongs is called mind, and a substance, whose immediate substratum is extension, is called body. Since thought and extension are distinct from each other, and since mind cannot only be known without the attributes of the body, but is in itself the negation of those attributes, we may say that the essence of these substances is in their reciprocal negation. Mind and body are wholly distinct, and have nothing in common.

8. We pass by the physics of Descartes, which has only a subordinate philosophical interest, and notice next his views of anthropology. From this dualistic relation between mind and matter, there follows a dualistic relation between soul and body. If matter is essentially extension, and mind essentially thought, and if the two have nothing in common, then the union of soul and body can be conceived only as a mechanical one. The body is to be regarded as an artistic automaton, which God has made, as a statue or machine formed by God from the earth. Within this body the soul dwells, closely but not internally connected with it. The union of the two is only a powerful bringing of the two together, since each is not only an independent factor, but is essentially distinct from and even opposed to the other. The body by itself is a machine fully prepared, in which nothing is changed by the entrance of the thinking soul, except that through it certain motions are secured: the wheel-work of the machine remains as it was. It is only thought which distinguishes this machine from every other; hence, therefore, brutes which are not self-conscious nor thinking, must be ranked with all other machines. From this standpoint arose especially the question concerning the seat of the soul. If body and soul are independent substances, each essentially opposed to the other, they cannot interpenetrate each other, but can touch only at one point when they are powerfully brought together. This point where the soul has its seat, is, according to Descartes, not the whole brain but the pineal gland, a little kernel in the middle of the brain. The proof for this claim, that the pineal gland is the only place where the soul immediately exhibits its energy, is found in the circumstance that all other parts of the brain are twofold, which should not be in an organ where the soul has its seat, else objects would appear double. There is, therefore, no other place in the body where impressions can be so well united as in this gland. The pineal gland is, therefore, the chief seat of the soul, and the place where all our thoughts are formed.

We have thus developed the fundamental thoughts of the Cartesian system, and will now recapitulate in a few words the features characteristic of its standpoint and historic position. Descartes was the founder of a new epoch in philosophy, first, from his postulate of universal freedom from all preconceptions. His protesting against every thing which is not posited by the thought, against taking any thing for granted in respect of the truth, has remained from that time onward the fundamental principle of the new age. Secondly. Descartes has brought out the principle of self-consciousness (the mind or the thinking substance is regarded by him as an individual self, a particular Ego)—a new principle, unknown in this view to the ancients. Thirdly. Descartes has shown the opposition between being and thought, existence and consciousness, and the mediation of this opposition, which has been the problem of the whole modern philosophy, he first affirmed as the true philosophical problem. But with these ideas, which make an epoch in the history of philosophy, there are at the same time connected the defects of the Cartesian philosophizing. First. Descartes gained the content of his system, namely his three substances, empirically. True, the system which begins with a protestation against all existence would seem to take nothing for granted, but to derive every thing from the thinking. But in fact this protesting is not thoroughly carried out. That which seems to be cast aside is afterwards, when the principle of certainty is gained, taken up again unchanged. And so it happens that Descartes finds at hand not only the idea of God, but his two substances as something immediately given. True, in order to reach them, he abstracts every thing which lies immediately before him, but in the end the two substances are seen as that which remains when all else is abstracted. They are received empirically. The second defect is, that Descartes separates so wholly from each other the two sides of the opposition between thought and being. He posits both as “substances,” i. e. as powers, which reciprocally exclude and negate each other. The essence of matter according to him consists only in extension, i. e. in the pure being extra se (Aussersichsein), and that of mind only in thought, i. e. in the pure being in se (Insichsein.) The two stand over against each other as centrifugal and centripetal. But with this apprehension of mind and matter, an inner mediation of the two is an impossibility; there must be a powerful act of creation, there must be the divine assistance in order that the two sides may ever come together, and be united as they are in man. Nevertheless Descartes demands and attempts such a mediation of the two sides. But the impossibility of truly overcoming the dualism of his standpoint is the third, and the chief defect of his system. In the proposition “I think, therefore I am,” or “I am thinking,” the two sides, being and thought, are indeed connected together, but only that they may become fixed independently in respect of each other. If the question is asked, how does the Ego stand related to the extended? the answer can only be: by thinking, i. e. negatively, by excluding it. The idea of God, therefore, is all that remains for the mediation of these two sides. The two substances are created by God, and through the divine will may be bound together; through the idea of God, the Ego attains the certainty that the extended exists. God is therefore in a certain degree a Deus ex machina, necessary in order to mediate the conflict of the Ego with the extended. It is obvious how external such a mediation is.

This defect of the Cartesian system operated as an impelling motive to those which succeeded.