Descartes using the method of doubt, tries to create a firm foundation for new sciences. This method should be based on something that is not susceptible to doubt. When Descartes applies the method of doubt, he doubts everything that are derived from senses. Descartes adopted the strategy of withholding his belief from anything that was not entirely certain and in dubitable. He decides that he cannot be deceived about his own existence, because if he did not exist, he would not be around to worry about it. If he did not exist, he would not be thinking; so if he is thinking, he must exist. This is the Cogito ergo sum, "I think therefore I am." Thinking comes first, and for Descartes that is a real priority. "The mind is better known than the body", and the cogito ergo sum makes Descartes believe, not just that he has proven his existence as a thinking substance, a mind, leaving the body as some foreign thing to worry about later. Descartes takes "I" as his first principle and thus "I think" is qualitatively different from our physical body. Thinking causes awareness and when we think we are conscious. Consciousness is very complete in itself for Descartes and it is unbroken, has no gaps. Freud challenges Descartes and says consciousness is less reliable than we think. Human beings cannot learn everything from consciousness. .
The essence for soul to Descartes, the attribute that makes a soul what is it, is thinking. "But what then am I? A thing that thinks. What is that? Athing that doubts, understands, affrims, denies, is willing, is unwilling, and also imagines and has sensory perceptions."(Descartes Medidation #2). Descartes" main concern is the mind-body problem. He claimed that human beings are composites of two kinds of substances, mind and body. A mind is a conscious thinking being, that is, it understands, wills, senses, and imagines. A body is a being extended in length, width, and breadth.