What did Descartes mean by "clear and distinct ideas"?

Descartes thought that there was a "natural light" of reason by which one could be sure of one's thoughts. Descartes wrote in his Principles of Philosophy (1644):

I term that "clear" which is present and apparent to an attentive mind, in the same way that we see objects clearly when, being present to the regarding eye, they operate upon it with sufficient strength. But the "distinct" is that which is so precise and different from all other objects that it contains within itself nothing but what is clear."

In other words, the thinker has an intuitive or direct experience of clarity and about what he or she is clear about. Descartes was relying on our ability to recognize when we know something for sure in all its detail.

What was the purpose of Descartes' Meditations?

In his Preface and Introduction to Meditations on First Philosophy (1641), Descartes said that his goal was to rationally prove the existence of God and the immortality of the soul. He claimed to be able to do that using his method of clear and distinct ideas, which would also enable him to create foundations of certainty for the sciences.

What are some of the major philosophical arguments made in Meditations?

Descartes believed it was necessary to take the entire edifice of knowledge down to its foundations to remove existing error. His method was not to doubt everything for the sake of skepticism itself, but to doubt everything that could be doubted, so that one would be left only with what was certain. He began with the usual arguments about the errors of the senses: for instance, the observation that far away objects look smaller than they are.

He then questioned whether he could be sure that there was a world outside of his mind and noted that the insanity of that line of questioning was not unusual if one takes into account the fact that every night, during sleep, there are bizarre distortions in dreams. This raises the question of what exactly is the difference between being awake and being asleep. Descartes notes that there is nothing in the quality of either experience that guarantees which state one is in.

Descartes' project of doubt next addresses mathematical and logical thinking. Descartes said that our confidence in these processes depends on our confidence that there is a benevolent God who guarantees that what seems self-evident to us really is true, and who guarantees the accuracy of the memory of those past thought processes that are necessary to proceed to a conclusion in a chain of reasoning.

Then, Descartes advances to his most devastating level of doubt: what if there is not a benevolent, all-powerful God, but an evil demon, who instead of supporting our true mental processes, is in fact constantly deceiving us about the workings of our own minds? So now Descartes has raised doubt to the level of doubting the existence of a good and powerful God, which he himself regards as a very disturbing and distressing predicament.

How did Descartes solve his evil demon hypothesis?

René Descartes recounted everything that he could doubt—sensory information, the external world, his own thought processes, and the goodness of God—and noted that one thing he could not doubt was that he himself was doing the doubting. From this he concluded that he could not doubt that he existed, since someone or something must be doing the doubting.


Descartes' assertion that he existed led to other conclusions, such that God exists as does the external world (iStock).

He wrote later about his famous cogito ergo sum, or "I think, therefore I am":

I noticed that while I was trying to think everything false, it must needs be that I, who was thinking this, was something. And observing that this truth, I am thinking, therefore I exist was so solid and secure that the most extravagant suppositions of the skeptics could not overthrow it, I judged that I need not scruple to accept it as the first principle of the philosophy that I was seeking.

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