René Descartes, answered from the texts and cited to the page.
The doubt was not destruction for its own sake. It was the only road to a foundation that could not be shaken. Consider what any careful enquirer finds when he examines the opinions he has received since childhood: they rest, most of them, on the testimony of the senses, and the senses have deceived him before. Once I had noticed that, the question became not which particular beliefs were false, but whether the entire edifice built on sensory testimony could be trusted.
The answer was that it could not, not yet, and so the method required suspending assent to whatever admitted of the least doubt, treating it provisionally as though it were false, in order to discover whether anything remained that could not be so treated.1 The doubt extends further than most expect. Even the demonstrations of mathematics fall under it, for we might ask whether some powerful being created us in such a condition that we are deceived even in what seems most evident; and since our being occasionally deceived is something experience already confirms, our being always deceived seems no less possible.2
Free will, however, gives us the means to withhold assent from what is not certain, and so to avoid being swept into error by the very faculties we cannot yet trust.3 Bourdin, in the Seventh Objections, pressed the charge that this procedure closes every avenue to truth — that if I renounce all doubtful things, even temporarily, I misuse them in the very act of renouncing them, and that whatever column I hope to erect leans so heavily on the temporary scaffolding of the doubt that it will fall when the scaffolding is removed.4
The charge mistakes the purpose entirely. The law of the method, properly understood, is simply this: in investigating what is certain, rely on nothing that is not certain.5 That is not a paradox; it is the condition any honest enquirer already accepts. The doubt is not a permanent renunciation but a single exercise, like the architect's scaffolding, used to hoist the column into place and then set aside.
And the column, once hoisted, stands on its own. From the universal doubt, which is like a fixed and unchangeable point, I resolved to derive the knowledge of God, of myself, and of all that the world contains.6 The doubts that alarmed Polyander at the outset are like phantoms that appear by the uncertain glimmer of a feeble light: fear pursues you if you flee, but if you approach and touch them, you find nothing but wind and shadow.7
What cannot be doubted survives the encounter; what collapses under it was never a proper foundation. The cogito is what survives — for we cannot conceive that what thinks does not at the same time as it thinks, exist, and hence this conclusion, I think, therefore I am, is the first and most certain of all that occurs to one who philosophises in an orderly way.8
That is why the doubt had to be total. A partial doubt, one that left some corner of the old opinions untouched, would have left the foundations mixed, some tested and some not. The once-in-a-lifetime exercise of pushing the doubt to its furthest extent is the price of a philosophy that rests on nothing it has not itself examined.
Reply 1. Assume the meaning of the law, 'in the investigation of the truth that which is to the slightest extent doubtful is to be treated as though it were false, to be: when we are investigating matters that are certain, we ought not to rely on anything which is not certain or is to the slightest extent doubtful.'The Philosophical Works of Descartes Vol II - Objections and Replies
We shall also doubt of all the other things which have formerly seemed to us quite certain, even of the demonstrations of mathematics and of its principles which we formerly thought quite self-evident... we have been told that God who created us can do all that He desires. For we are still ignorant of whether He may not have desired to create us in such a way that we shall always be deceived, even in the things that we believe ourselves to know best.The Philosophical Works of Descartes Vol I
But meanwhile whoever turns out to have created us, and even should he prove to be all-powerful and deceitful, we still experience a freedom through which we may abstain from accepting as true and indisputable those things of which we have not certain knowledge, and thus obviate our ever being deceived.The Philosophical Works of Descartes Vol I
look out lest your pedestal and column lean so much upon your temporary scaffolding that they will fall if you remove it. It is this that seems to merit censure in your method. It reposes on false foundations, and it leans upon them so much that if they are removed it itself falls to the ground.The Philosophical Works of Descartes Vol II - Objections and Replies
when we are investigating matters that are certain, we ought not to rely on anything which is not certain or is to the slightest extent doubtful. In this case your law is quite sound, is of established usage, and one of the best known truths common to all Philosophers.The Philosophical Works of Descartes Vol II - Objections and Replies
it is really from this universal doubt which is like a fixed and unchangeable point, that I have resolved to derive the knowledge of God, of yourself, and of all that the world contains.The Philosophical Works of Descartes Vol I
those doubts which alarmed you to begin with, are like those phantoms and vain images which appear in the night by the uncertain glimmer of a feeble light. Fear pursues you if you flee, but if you approach and touch them, you will find nought but wind and shadow.The Philosophical Works of Descartes Vol I
we cannot in the same way conceive that we who doubt these things are not; for there is a contradiction in conceiving that what thinks does not at the same time as it thinks, exist. And hence this conclusion I think, therefore I am, is the first and most certain of all that occurs to one who philosophises in an orderly way.The Philosophical Works of Descartes Vol I