cogito (usually ‘the cogito’) The principle of establishing the existence of a being from the fact of its thinking or awareness. https://en.oxforddictionaries.com/definition/cogito
“This proposition became a fundamental element of Western philosophy, as it purported to form a secure foundation for knowledge in the face of radical doubt. While other knowledge could be a figment of imagination, deception, or mistake, Descartes asserted that the very act of doubting one’s own existence served—at minimum—as proof of the reality of one’s own mind; there must be a thinking entity—in this case the self—for there to be a thought.”
Some people consider the wide acceptance of this philosophic statement as a turning point in Western thinking and subsequent behaviour. This includes a sense of superiority of humans and rational thought, and an indifference towards the natural world and creative thinking. This informs our present situation of atheism and religious fundamentalism, environmental exploitation and planetary destruction.
There are other considerations, including earlier thinkers who explored similar ideas and various philosophic challenges. Regardless of these, Descartes put the individual thinking ego front and centre in consciousness and not long thereafter, Nietzsche remarked that ‘God is dead’.
So that seems to have all worked out ok then.