Descartes' Method: Doubt, Four Rules, and the Cogito

Descartes arrives at the problem of method by a concrete route. After the most thorough schooling his time could offer, he finds himself, by his own account, with no more than doubts about everything and a clear sense of how little he actually knows, as the Stanford Encyclopedia of Philosophy reports.

The problem of knowledge: tradition and the senses on trial

Scholastic tradition, built on Aristotle, held that every person is born with an empty mind and that all the material of intellectual knowledge must come from the senses, according to the Internet Encyclopedia of Philosophy. It is the same theoretical frame that, centuries earlier, had driven early Greek thinkers to search for the arché, the first principle of things: even then, philosophers were looking for a secure foundation from which every other piece of knowledge could follow.

Descartes turns that starting point around. If knowledge comes from the senses, and the senses can sometimes deceive, then their reliability as a source of knowledge becomes doubtful: this is the position the Internet Encyclopedia of Philosophy attributes to him. It traces the opening move of the Cartesian project to the decision to clear the mind of every opinion accepted before, ahead of rebuilding anything new.

Practical example: imagine learning to find your way home by sight alone, at night, with little light. If one evening you mistake a shadow for a step and fall, your senses have deceived you once — that is enough to doubt they are always a safe guide, even if they had never failed you before.

Questioning belief by belief, stripping away one claim at a time until something holds firm, recalls from a distance how Socrates tested his interlocutors’ definitions, question after question, to see which ones survived.

Methodic doubt

The Stanford Encyclopedia of Philosophy calls Descartes’ doubt “methodic,” and notes that it reaches beyond knowledge drawn from the senses alone. The Internet Encyclopedia of Philosophy describes the basic strategy in precise terms: treat as false any belief that can be doubted even slightly. The filter rejects anything doubtable even a little, and keeps only what survives the next round of questioning.

Descartes’ doubt aims at a final certainty, a different use from the one often attributed to the Sophists, who in fifth-century Athens were said to give up looking for a shared truth.

The Discourse on Method: four rules from 1637

According to the Stanford Encyclopedia of Philosophy, the Discourse on Method also served a pedagogical and demonstrative purpose, and the method it describes was meant, for Descartes, to yield knowledge useful to life. The same encyclopedia, in its entry on Descartes’ method, lists four rules set out in Part II of the work.

The first rule calls for accepting as true nothing that is not known with clear evidence. The second calls for dividing every difficulty under examination into as many parts as possible, the better to resolve it. The third calls for conducting thoughts in order, starting from the simplest and easiest objects to know and rising gradually toward knowledge of the most complex. The fourth, finally, calls for reviews so complete and exhaustive that one can be certain nothing has been left out.

Together the four rules form a general procedure for guiding reason, meant to apply beyond the metaphysical questions where it first takes shape.

From doubt to the first certainty: the cogito

In the Meditations, Descartes notes that everything he had accepted as most true had come to him from the senses or through the senses, hence the need to doubt that source of knowledge as well, according to the Stanford Encyclopedia of Philosophy. Pushing methodic doubt to its limit, Descartes goes so far, in the second Meditation, as to imagine a deceiving demon able to falsify everything else: not even that demon, the Stanford Encyclopedia of Philosophy notes, could make him believe he is nothing, so long as he thinks he is something.

From this core of reasoning the cogito follows. Descartes concludes that the very act of thinking, in the moment it happens, is enough to make him certain he exists: that is how the Internet Encyclopedia of Philosophy puts it. The Stanford Encyclopedia of Philosophy reports the Latin formula “Ego cogito, ergo sum,” which Descartes himself calls a proposition necessarily true whenever it is stated or conceived in the mind. For the Stanford Encyclopedia of Philosophy, Descartes treats the cogito as certain because the intellect perceives it clearly and distinctly.

The cogito is a partial endpoint in the Cartesian project: the first stone that methodic doubt cannot dislodge. From there the process starts over to rebuild everything else.

Anyone tracing the question of a foundation for knowledge back further, into Greek philosophy, finds an echo of it in Plato’s theory of forms, which places true knowledge in an order distinct from the sensible world, beyond the reach of the senses. The intellectual climate in which Descartes was formed, in turn, is the one the Renaissance had already shaken a century earlier, calling inherited authorities into question across more than one field of knowledge.