While examining his nature in the Second Meditation, Descartes famously finds that he is “in the strict sense only a thing that thinks” (res cogitans), that thought is inseparable from him, and that the nature of his self or mind (or soul) consists in thinking alone (AT VII 27, CSM II 189). In the new, broad sense of the term here introduced, thinking covers not only functions traditionally attributed to mind, intellect, or reason, such as judging, understanding, conceiving, and reasoning, but any kind of cognitive and conative act, like doubting, affirming, denying, willing, and not willing, as well as imagining and having sensory perceptions (AT VII 28, CSM II 19). In addition to those of the rational soul, all of the functions of what in the Aristotelian tradition were attributed to the animal and sensitive soul are attributed to the mind in a new, broad sense and treated as different kinds or modes of thought. The traditional architecture of mind and cognition has been, it seems, reversed. Instead of serving as the foundation required for the intellect to do its work of abstraction and understanding, sensory perception, although also requiring bodily organs, now depends on the mind and its capacity to think and understand.
One can roughly distinguish two main strands for later theorizing about mind and thought that can be traced to the Meditations and are likely to color one's understanding of the notion of thought defining the being discovered in the Second Meditation. One grows out of the empiricist tradition and emphasizes consciousness as distinctive for or even identical with thought or mind. The focus here, using a recent slogan, is on what the mind is, and on what a particular thought is ( or feels) like, as opposed to what the mind does or how it acts and operates (Chalmers 1996, Brandom 2002). The second strand with roots in the Aristotelian-Scholastic tradition focuses on what mind does and sees the exercise of its various intellectual capacities and the standards governing them as essential to thought. It lives on from Descartes to Kant and beyond to those who take their inspiration from Kant rather than from the British empiricists in their accounts of cognition and the relation between thought and the world.