René Descartes (March 31, 1596 – February 11, 1650) was a highly influential French philosopher, mathematician, physicist and writer. He is known for his influential arguments for substance dualism, where mind and body are considered to have distinct essences, one being characterized by thought, the other by spatial extension. He has been dubbed the "Father of Modern Philosophy" and the "Father of Modern Mathematics." He is also known as Cartesius.
- Dubium sapientiae initium.
- Cogito, ergo sum.
- I think, therefore I am.
- Variant: I think therefore I exist.
- Principia philosophiae (Principles of Philosophy) (1644), Part I, Article 7
- Da ei verpam.
- Attested in Clarke, Desmond M. (2006). Descartes : a Biography. Cambridge Press. p. 67. ISBN 978-0-521-82301-2.
- Ex nihilo nihil fit.
- Nothing comes out of nothing.
- Principia philosophiae, Part I, Article 49
- Me tenant comme je suis, un pied dans un pays et l’autre en un autre, je trouve ma condition très heureuse, en ce qu’elle est libre.
- Staying as I am, one foot in one country and the other in another, I find my condition very happy, in that it is free.
- Letter to Elisabeth of Bohemia, Princess Palatine (Paris, June/July 1648)
- If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.
- Darling, David J. (2004). The Universal Book of Mathematics. Wiley. p. 90. ISBN 978-0-471-27047-8.
- Variant: If you would be a real seeker after truth, you must at least once in your life doubt, as far as possible, all things.
- Variant: In order to seek truth, it is necessary once in the course of our life, to doubt, as far as possible, of all things.
- John, Veitch (2009). The Meditations and Selections from The Principles of Rene Descartes. BiblioLife, LLC. p. 130. ISBN 978-1-110-27664-6.
- So blind is the curiosity by which mortals are possessed, that they often conduct their minds along unexplored routes, having no reason to hope for success, but merely being willing to risk the experiment of finding whether the truth they seek lies there.
- Rules for the direction of the mind: IV
Le Discours de la Méthode (1637) 
- Discourse on the Method of Rightly Conducting the Reason, and Seeking Truth in the Sciences
- Of all things, good sense is the most fairly distributed: everyone thinks he is so well supplied with it that even those who are the hardest to satisfy in every other respect never desire more of it than they already have. fr en
- Pt. 1
- Good sense is of all things in the world the most equally distributed, for everybody thinks he is so well supplied with it, that even those most difficult to please in all other matters never desire more of it than they already possess.
- Common sense is the best distributed commodity in the world, for every man is convinced that he is well supplied with it.
- Nothing is more fairly distributed than common sense: no one thinks he needs more of it than he already has.
- It is not enough to have a good mind. The main thing is to use it well.
- Pt. 1
- The greatest minds, as they are capable of the highest excellencies, are open likewise to the greatest aberrations; and those who travel very slowly may yet make far greater progress, provided they keep always to the straight road, than those who, while they run, forsake it.
- Pt. 1
- The greatest minds are capable of the greatest vices as well as of the greatest virtues.
- The first precept was never to accept a thing as true until I knew it as such without a single doubt.
- Pt. 2
- Divide each difficulty into as many parts as is feasible and necessary to resolve it.
- Pt. 2
- The last rule was to make enumerations so complete, and reviews so comprehensive, that I should be certain of omitting nothing.
- Pt. 2
- The long chains of simple and easy reasonings by means of which geometers are accustomed to reach the conclusions of their most difficult demonstrations, had led me to imagine that all things, to the knowledge of which man is competent, are mutually connected in the same way, and that there is nothing so far removed from us as to be beyond our reach, or so hidden that we cannot discover it, provided only we abstain from accepting the false for the true, and always preserve in our thoughts the order necessary for the deduction of one truth from another.
- Pt. 2
- Each problem that I solved became a rule, which served afterwards to solve other problems.
- Pt. 2
- One cannot conceive anything so strange and so implausible that it has not already been said by one philosopher or another.
- Variant: There is nothing so strange and so unbelievable that it has not been said by one philosopher or another.
- Pt. 2
- Je pense, donc je suis.
- I think, therefore I am.
- Pt. 4
- [B]ut if there were machines bearing the image of our bodies, and capable of imitating our actions as far as it is morally possible, there would still remain two most certain tests whereby to know that they were not therefore really men.
- Pt. 5
Quotes about Descartes 
- Aristotle remarks in his Poetics that poetry is superior to history, because history presents only what has occurred, poetry what could and ought to have occurred, poetry has possibility at its disposal. Possibility, poetic and intellectual, is superior to actuality; the esthetic and the intellectual are disinterested. But there is only one interest, the interest in existing; disinterestedness is the expression for indifference to actuality. The indifference is forgotten in the Cartesian Cogito-ergo sum, which disturbs the disinterestedness of the intellectual and offends speculative thought, as if something else should follow from it. I think, ergo I think; whether I am or it is (in the sense of actuality, where I means a single existing human being and it means a single definite something) is infinitely unimportant. That what I am thinking is in the sense of thinking does not, of course, need any demonstration, nor does it need to be demonstrated by any conclusion, since it is indeed demonstrated. But as soon as I begin to want to make my thinking teleological in relation to something else, interest enters the game. As soon as it is there, the ethical is present and exempts me from further trouble with demonstrating my existence, and since it obliges me to exist, it prevents me from making an ethically deceptive and metaphysically unclear flourish of a conclusion.
- Soren Kierkegaard Concluding Unscientific Postscript, Hong P. 318-319
- I would inquire of reasonable persons whether this principle: Matter is naturally wholly incapable of thought, and this other: I think, therefore I am, are in fact the same in the mind of Descartes, and in that of St. Augustine, who said the same thing twelve hundred years before. ...I am far from affirming that Descartes is not the real author of it, even if he may have learned it only in reading this distinguished saint; for I know how much difference there is between writing a word by chance without making a longer and more extended reflection on it, and perceiving in this word an admirable series of conclusions, which prove the distinction between material and spiritual natures, and making of it a firm and sustained principle of a complete metaphysical system, as Descartes has pretended to do. ...it is on this supposition that I say that this expression is as different in his writings from the saying in others who have said it by chance, as in a man full of life and strength, from a corpse.
- The truth is sum, ergo cogito — I am, therefore I think, although not everything that is thinks. Is not consciousness of thinking above all consciousness of being? Is pure thought possible, without consciousness of self, without personality? Can there exist pure knowledge without feeling, without that species of materiality which feelings lends to it? Do we not perhaps feel thought, and do we not feel ourselves in the act of knowing and willing? Could not the man in the stove [Descartes] have said: "I feel, therefore I am"? or "I will, therefore I am"? And to feel oneself, is it not perhaps to feel oneself imperishable?