Rene Descartes, in his book, Meditations on First Philosophy (1641) believed that knowledge can proceed or start from very few premises or starting points just like his model of mathematics. Once one is certain about what these starting points of knowledge are, he can expand it. He saw the structure of knowledge as an inverted pyramid where a few premises are the starting points. For something to be accepted as a starting point of knowledge, an idea must be clear, distinct, and certain. He believed that reason is an important instrument or source in gaining knowledge about reality because it is reason alone, which can discover the indubitable premises of knowledge which are self-evidently true and where other propositions could be deduced, thus, knowledge could proceed.
The indubitable premises are logically true and non-sensical to doubt because the moment that you doubt them, you would contradict yourself. These clear and distinct ideas which Descartes considered as the starting points of knowledge: the self, God, and material objects. Hence, they became the three indubitable premises of knowledge. These three could be discovered using the method of systematic doubt. They are considered as substances, where, through the use of the transcendental faculty of reason, one would be able to gain knowledge of, without having to rely on experience as a source of knowledge.
Let us take Descartes’ proof for the existence of the self as an example of the use of his method of systematic doubt. The method of systematic doubt consists of doubting everything that can be doubted until you arrive at clear and distinct ideas which are non-sensical to doubt. For something to be accepted as one of the starting points or premises of knowledge, an idea must be clear and distinct. Descartes believed that one of the logically certain premises is the existence of the self He began his proof for the existence of the self by doubting everything that can be doubted. Even if you doubt everything that can be doubted (e.g. you can doubt your parents, if they are really your parents; or your brothers and sisters, if you are really related by blood; or even the existence of things in the other room, if nobody is there to perceive them; etc., as Descartes argues that you can even doubt your own doubt!), you can still be sure and certain about one thing, that you are doubting. Doubting is a form of thinking. Thinking could not happen in a vacuum. There must be an owner of these thoughts. Therefore, thinking implies that you exist as a substance. You would be contradicting yourself if you doubt that you exist, at the very moment that you are doubting. As a result, the self exists at the very act of doubting, which is a form of thinking. Or in other words, cogito, ergo, sum or think, therefore, I am.’
Moreover, the existence of the self as a substance is independent of the body. The self, as a substance, exists at its own nature and has an independent existence. It exists on its own without being dependent on the existence of the body. Thus, according to Descartes, he could imagine himself existing without the body, but he could not imagine himself existing without the mind. He then went on to use the method of systematic doubt to prove the two other indubitable premises of knowledge, the existence of God and material objects, as infallible knowledge of infallible — incapable of substances guaranteed by the faculty of reason. making mistakes or being Some critics, on the other hand, could not accept Descartes’ argument. For one, wrong there was already the ‘I’ that does the doubting and the thinking right from the start. The self, that he concluded to exist, was right there at the beginning of his argument as an assumption or premise. I doubt (dubito); I think (cogito); therefore, I exist (existo). Another point raised against him was that he assumed that someone has to perceive things and he is different from the things he perceives—that is, the knower is a different or separate entity from the known. But you are not sure whether or not you are actually different from your perceptions and experiences because there is a possibility that the self is a sum of perceptions and experiences that you have formed from birth. This is the claim of empiricism which contradicts the rationalism of Descartes.
Despite the criticisms hurled against him, Descartes’ legacy of skepticism as a method in philosophy should not be ignored. This process of doubting or a little amount of skepticism would later on leave us with a very important life lesson. That is, of never accepting the truth of any statement or belief without adequate evidence for it. This process of doubting could spell the difference between being a gullible individual that could be easily fooled and a level-headed person with a knack for investigation and the habit of looking for evidence before believing in something.