Encyclopedia of Philosophy: THE ESSENCE OF MIND

In the rest of the Meditations Descartes proceeds to answer the question ‘What am I, this I whom I know to exist?' The immediate answer is that I am a thing which thinks (res cogitans). ‘What is a thing which thinks? It is a thing which doubts, understands, conceives, affirms, denies, wills, refuses, which also imagines and feels.' ‘Think' is being used in a wide sense: for Descartes, to think is not always to think that something or other, and thinking includes not only intel¬lectual meditation, but also volition, emotion, pain, pleasure, mental images, and sensations. No previous author had used the word with such a wide extension. But Descartes did not believe that he was altering the sense of the word: he applied it to the new items because he believed that if they were properly under¬stood, they could be seen to possess the feature which was the most important characteristic of the traditional items if they were properly understood. This fea¬ture was immediate consciousness, which for him was the defining feature of thought. ‘I use this term to include everything that is within us in such a way that we are immediately conscious of it. Thus, all the operations of the will, the intellect, the imagination and the senses are thoughts.'