Methodical Doubt (or Cartesian Doubt) is a philosophical method developed by the philosopher René Descartes. The question that preoccupied Descartes in his books Meditations and Discourse On Method was how to obtain secure knowledge and how to know whether what one has learned is correct. Methodical Doubt initially consists of doubting everything, or in other words that everything should be doubted. The purpose of Descartes' doubt is to arrive at something which cannot be doubted and which is therefore certain knowledge. The certain knowledge that Descartes initially arrives at is that he exists because he thinks: cogito, ergo sum ("I think, therefore I am"). Even when he doubts everything else, he can not doubt his own existence.
Rules of Methodical Doubt
Descartes was thorough in his search for a sure insight. He set out four methodological rules for systematically doubting:
1.To begin with, one must consider it only as true, which appears clearly and distinctly to the consciousness and to the senses. For example, it is clear that a triangle has only three sides and that a sphere has only one surface.
2. Each problem must be divided into as many sub-problems as possible. A sub-problem is easier to solve than the whole problem. For example, if you want to find out how a house is built, it does not help much if you stand outside and look at the whole house. You get a better understanding if you go into the house and examine each floor separately.
3. One should start with the elements which are the simplest. Then one has to gradually move on to the more complicated elements. For example, one should not start by examining whether the double of 32 is 64. One should start by examining whether the double of 1 is 2. Once one has found out that this is the case, one can join, that the double of 2 is 4 and so on...
4. One should make a count of all the conditions that are relevant to the problem one is investigating. Otherwise, our knowledge will not be secure and complete. On the other hand, you should only count the conditions that are relevant to the solution of the problem and do not include anything superfluous in your count. For example, if you want to examine whether the area of a circle is larger than all other figures with the same circumference, then you do not need to examine all conceivable figures. One only needs to examine a limited number of figures. Then one can agree whether this rule applies to all other figures that have the same circumference as the circle.
See also: Descartes' method of doubt