Rene Descartes’ famous assertion from the Discourse on Method, part 4, and restated in the Second Meditation. Descartes' method of introspection leads him to believe that even if all else is dubitable, it cannot be the case that as long as he is conscious (i.e., has a mental life) he does not exist. Thus, awareness alone is sufficient to guarantee his existence.
He will go on to investigate the nature of this belief and argue it is distinct from other kinds of beliefs in that because of its indubitability, and, therefore, knowledge of the mind’s (i.e., soul’s) existence is more certain than that of the body. Hence, the mind cannot be identical to the body (i.e., brain). This “proof” for the soul’s independent existence was intended by Descartes to entice the Catholic intelligentsia to adopt his new method as a replacement for Scholastic Aristotelianism in the Catholic universities of Europe. His arguments were not well received because his methodology shifted the weight of authority from church authority and divine revelation to the power of human reason. During the Counter Reformation his works would be included in the Index Librorum Prohibitorum (i.e., list of banned books).