Rene Descartes

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

If we possessed a thorough knowledge of all the parts of the seed of any animal (e.g. man), we could from that alone, be 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.

Mathematics is a more powerful instrument of knowledge than any other that has been bequeathed to us by human agency.

Before examining this more carefully and investigating its consequences, I want to dwell for a moment in the contemplation of God, to ponder His attributes in me, to see, admire, and adore the beauty of His boundless light, insofar as my clouded insight allows. Believing that the supreme happiness of the other life consists wholly of the contemplation of divine greatness, I now find that through less perfect contemplation of the same sort I can gain the greatest joy available in this life.

I am accustomed to sleep and in my dreams to imagine the same things that lunatics imagine when awake.

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

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

I have concluded the evident existence of God, and that my existence depends entirely on God in all the moments of my life, that I do not think that the human spirit may know anything with greater evidence and certitude.

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

The only secure knowledge is that I exist.

The chief cause of human errors is to be found in the prejudices picked up in childhood.

For to be possessed of a vigorous mind is not enough; the prime requisite is rightly to apply it.

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.

But I cannot forget that, at other times I have been deceived in sleep by similar illusions; and, attentively considering those cases, I perceive so clearly that there exist no certain marks by which the state of waking can ever be distinguished from sleep, that I feel greatly astonished; and in amazement I almost persuade myself that I am now dreaming.

Whatever I have up till now accepted as most true and assured I have gotten either from the senses or through the senses. But from time to time I have found that the senses deceive, and it is prudent never to trust completely those who have deceived us even once.

Now therefore, that my mind is free from all cares, and that I have obtained for myself assured leisure in peaceful solitude, I shall apply myself seriously and freely to the general destruction of all my former opinions.

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.

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?

We never understand a thing so well,and make it our own, as when we have discovered it for ourselves.

Desire awakens only to things that are thought possible.

Be that as it may, there is fixed in my mind a certain opinion of long standing, namely that there exists a God who is able to do anything and by whom I, such as I am, have been created. How do I know that he did not bring it about that there is no earth at all, no heavens, no extended thing, no shape, no size, no place, and yet bringing it about that all these things appear to me to exist precisely as they do now?