One cannot conceive anything so strange and so implausible that it has not already been said by one philosopher or another.

Rene Descartes

[About Pierre de Fermat] It cannot be denied that he has had many exceptional ideas, and that he is a highly intelligent man. For my part, however, I have always been taught to take a broad overview of things, in order to be able to deduce from them general rules, which might be applicable elsewhere.

intelligent men science

Even if I were to suppose that I was dreaming and whatever I saw or imagined was false, yet I could not deny that ideas were truly in my mind.

mind dream ideas

Here I beg you to observe in passing that the scruples that prevented ancient writers from using arithmetical terms in geometry, and which can only be a consequence of their inability to perceive clearly the relation between these two subjects, introduced much obscurity and confusion into their explanations.

experience confusion two

Common sense is the most fairly distributed thing in the world, for each one thinks he is so well-endowed with it that even those who are hardest to satisfy in all other matters are not in the habit of desiring more of it than they already have.

common-sense respect thinking

I accept no principles of physics which are not also accepted in... by Rene Descartes

I accept no principles of physics which are not also accepted in mathematics.

accepted physics principles

If I go for the alternative which is false, then obviously I shall be in error; if I take the other side, then it is by... chance that I arrive at the truth, and I shall still be at fault.... In this incorrect use of free will may be found the privation which constitutes the essence of error.

errors essence may

For each of us there is a set limit to our intellectual powers wh... by Rene Descartes

For each of us there is a set limit to our intellectual powers which we cannot pass.

hard-work encouragement intellectual

It must not be thought that it is ever possible to reach the interior earth by any perseverance in mining: both because the exterior earth is too thick, in comparison with human strength; and especially because of the intermediate waters, which would gush forth with greater impetus, the deeper the place in which their veins were first opened; and which would drown all miners.

perseverance water science

If I found any new truths in the sciences, I can say that they follow from, or depend on, five or six principal problems which I succeeded in solving and which I regard as so many battles where the fortunes of war were on my side.

battle math war

Archimedes, that he might transport the entire globe ... demanded only a point that was firm and immovable; so also, I shall be entitled to entertain the highest expectations, if I am fortunate enough to discover only one thing that is certain and indubitable.

enough expectations might

Good sense is, of all things among men, the most equally distributed; for every one thinks himself so abundantly provided with it, that those even who are the most difficult to satisfy in everything else, do not usually desire a larger measure of this quality than they already possess.

desire men thinking

I concluded that I might take as a general rule the principle that all things which we very clearly and obviously conceive are true: only observing, however, that there is some difficulty in rightly determining the objects which we distinctly conceive.

principles math might

There is a difference between happiness, the supreme good, and the final end or goal toward which our actions ought to tend. For happiness is not the supreme good, but presupposes it, being the contentment or satisfaction of the mind which results from possessing it.

differences goal mind

It is contrary to reasoning to say that there is a vacuum or space in which there is absolutely nothing.

contrary vacuums space

The entire method consists in the order and arrangement of the things to which the mind's eye must turn so that we can discover some truth.

The nature of matter, or body considered in general, consists not in its being something which is hard or heavy or coloured, or which affects the senses in any way, but simply in its being something which is extended in length, breadth and depth.

depth body science

Cogito, ergo, sum. (I think therefore I am.) by Rene Descartes

Cogito, ergo, sum. (I think therefore I am.)

It is only prudent never to place complete confidence in that by which we have even once been deceived.

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.

I know not if I was then a man dreaming I was a butterfly, or if I am now a butterfly dreaming I am a man.