In the Discourse on Method, he enumerated his method and the four rules which he considered essential for the success for any philosophical project:

1. To take nothing for granted. Everything would begin with and proceed by means of doubt, in order to avoid bias and prejudgement;
2. To split an argument into simple parts;
3. To proceed by degrees, from simple indubitable truths to more complex ideas;
4. To evaluate often in order to ensure that nothing is missed and that the whole argument is kept under review.

Descartes suggested that the last of these rules is most important: enumerations need to be so complete and reviews so general that nothing is omitted. It is necessary to show that any preferred alternative is only one possibility among many others, all of which deserve full consideration.

Descartes’ initial application of the method was to seek affirmation of his own existence through a logical, rational process. From this he deduced his famous cogito ergo sum argument:

I resolved to pretend that everything that had ever entered my mind was no more true than the illusions of my dreams. But inrmediately afterward I noted that, during the time I wanted thus to think that everything was false, it was necessary that I who thought thus, be something. And noticing that this trust – I think, therefore I am – was so firm and so certain that the most extravagant suppositions of the sceptics were unable to shake it, I judged that I would accept it without scruple as the first principle of the philosophy I was seeking. (Descartes, 1980, p. 17)

Classic rationalists, like Descartes, believed that we are more than the sum of our empirical interactions with our environment and that we are endowed with a “rational soul”. Apart from his method, one of Descartes’ other important contributions to philosophical thought is the notion of subjectivism. His argument was that if I can only know my own mind and its contents with any certainty, I cannot possibly have knowledge of other minds or material objects. As a result, everything outside of my own mind becomes problematic. Everything can only be proved to exist by inference from my own consciousness.

Locke (1632 – 1704)

John Locke was a significant figure in the English Enlightenment and was greatly influenced by Descartes. He was stimulated by Descartes’ rethinking of the foundations of knowledge. Despite coming to be associated with empiricism because of his emphasis on ‘sense experience’ as the source of knowledge, Locke maintained a strong respect for rationality. His approach is embedded in the methodological foundations of Socrates and Descartes – though his conclusions depart from theirs.

In his An Essay Concerning Human Understanding, Locke (1979) establishes two important premises. Firstly he argues that extreme scepticism is impractical, “If we will disbelieve everything, because we cannot certainly know all things, we shall do muchwhat [in Hume’s original usage, ‘muchwhat’ means ‘almost’] as wisely as he who would not use his legs, but sit still and perish, because he had no wings to fly” (ibid., Introduction, p. 5). Secondly, he argues at length that the concept of innate human knowledge or wisdom is untenable. In making these arguments, he effectively rejects Descartes’ attempt to create a perfectly rational understanding of the world Instead, he argues famously for the concept of the human mind as a tabula rasa, an “empty cabinet” furnished by the ideas entering into it through the senses (ibid., Book I, Chapter l, p. 15).