To know what people really think, pay regard to what they do, rather than what they say.

I think, therefore I am.

It is easy to hate and it is difficult to love. This is how the whole scheme of things works. All good things are difficult to achieve; and bad things are very easy to get.

Doubt is the origin of wisdom.

The reading of all good books is like a conversation with the finest minds of past centuries.

In order to improve the mind, we ought less learn than to contemplate.

Illusory joy is often worth more than genuine sorrow.

If you choose not to decide, you still have made a choice.

I hope that posterity will judge me kindly, not only as to the things which I have explained, but also to those which I have intentionally omitted so as to leave to others the pleasure of discovery.

Except our own thoughts, there is nothing absolutely in our power.

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.

Each problem that I solved became a rule, which served afterwards to solve other problems.

Divide each difficulty into as many parts as is feasible and necessary to resolve it.

Whenever anyone has offended me, I try to raise my soul so high that the offense cannot reach it.

When it is not in our power to determine what is true, we ought to follow what is most probable.

When I consider this carefully, I find not a single property which with certainty separates the waking state from the dream. How can you be certain that your whole life is not a dream?

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.

If we possessed a thorough knowledge of all the parts of the seed of any animal (e.g. man), we could from that alone, by reasons entirely mathematical and certain, deduce the whole conformation and figure of each of its members, and, conversely if we knew several peculiarities of this conformation, we would from those deduce the nature of its seed.

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.

I am indeed amazed when I consider how weak my mind is and how prone to error.

There is nothing so strange and so unbelievable that it has not been said by one philosopher or another.

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.

The greatest minds are capable of the greatest vices as well as of the greatest virtues.

To do is to be.

The long concatenations of simple and easy reasoning which geometricians use in achieving their most difficult demonstrations gave me occasion to imagine that all matters which may enter the human mind were interrelated in the same fashion.