I was especially delighted with the mathematics, on account of the certitude and evidence of their reasonings; but I had not as yet a precise knowled… - René Descartes

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

ART. 17. The functions of the soul Having thus considered all the functions that pertain to the body alone, it is easy to recognize that there remains nothing in us that we should ascribe to the soul except our thoughts, of which there are two main kinds: the actions of the soul and its passions. By its actions I mean all our volitions, since we experience these as coming directly from our soul, and as apparently depending on it alone. Whereas, on the other hand, the term ‘passions’ can be applied in general to all the kinds of perception or knowledge to be found within us, because it is often not our soul that makes them what they are, and because they are always produced in us by the things they represent.

[I]t seems to be just as foolish to say, 'I imagine, in order to understand more clearly what I am,' as to say, 'I am now clearly awake and I see something true, but because I do not yet see it clearly enough I shall fall asleep so that my dreams will represent it to me more truly and clearly.