This has become the motto of the modern mind. Descartes championed the mind-matter split. Much of our social and political paradigm stems from this dualism. From this dualism flow individualism, industrialism, humanism, capitalism and egoism.
Dualism also gives birth to fragmentation, separation, alienation and isolation.
That I exist in my thoughts and in my mind is only partially true and therefore is a flawed philosophy. My existence is a network of influences. I am because earth, air, fire and water are. I am because my parents, my teachers and my friends are. Shakespeare is, therefore I am.