Found in a letter…

[jadesolitude]

"Each nation is the more civilized and polished the better its members are versed in philosophy and, accordingly, that the greatest good which can exist in a State is to have true philosophers.  Besides this… as far as each individual is concerned, it is not only useful for him to live aong those who apply themselves to this study, but that it is incomparably better to apply himself to it; just, no doubt, as it is much better to use one's own eyes to guide onself, and to enjoy by the same means the beauty of colour and light, than to have them closed and to follow the lead of another, althought this latter course is better than to keep them closed and to have only oneself as a guide.  For to live without philosophizing is indeed tantamount to keeping one's eyes closed without ever trying to open them, while the pleasure of seeing all the things that our sight reveals is not to be compared to the satisfaction which comes from the knowledge of those discovered by philosophy.  And, finally, this study is more necessary for regulating our maners, and for guiding us in this life, than the use of our eyes is for directing our steps.  Brute beasts, which have only their bodies to preserve, are occupied continuously in seeking food; but men, whose principal part is the mind, should make the search for wisdom their main preoccupation, for wisdom is the true nourishment of the mind; and I am convinced, moreover, that there are many who would not fail to do so, if only they had hope of succeeding, and knew how far they are capable of it.  There is no soul, provided it has the least spark of nobility, which remains so attached to the objects of the senses, that it does not sometimes turn away from them to aspire to some greater good, even though it frequently does not know in what that good consists.  Those whom fortune favours most, who have an abundance of health, of honours, and of riches, are no more exempt from this desire than the others; on the contrary, I am pursuaded that it is they who yearn the most ardently after another good, more sovereign than all those which they possess.  But this sovereign good, considered by natural reason without the light of faith, is nothing other than the knowledge of truth through it's first causes, that is to say wisdom, of which philosophy is the study…"
-René Descartes (translation by F.E. Sutcliffe)

(later in the same letter:)
"…those who profess to being philosophers are often less wise and reasonable than others who have never applied themselves to this study…"