Questions such as; are my senses trust-worthy? What is my essential nature? All pertain to the major question of what constitutes as substance? This issue is the first fundamental conflict between the three philosophers. As Descartes was the first to touch on this problem, I will deal with him primarily. Descartes method of doubt, found in Meditations, was to doubt everything. He did not trust his senses, as they were prone to mistakes. He presented the idea that, as dreams can be so clear and vivid he doubted we could know whether we are dreaming or not. He also stated that we could not know whether our whole concept of reality was real as it could be fabricated by an evil demon. Then, it seems, everything must remain in doubt, not so for Descartes. He found that there is one thing that you can never be deceived on and that is; ‘you cannot doubt that you are doubting.’ You cannot be deceived that you are conscious or not as the very act of thinking you are conscious, is a conscious act. ‘Cogito Ergo Sum.’ I think therefore I am. Thus the mind becomes a core substance for Descartes.
Yet in