[…] the strangest nonsense becomes self-understood. To Descartes, as you know, is due the saying ‘I think, therefore I am.’
Countless clever thinkers have accepted this as true: ‘I think, therefore I am.’ Yet the result is this: From morning until evening I think, therefore I am. Then I fall asleep. I do not think, therefore I am not. I wake up again, I think, therefore I am. I fall asleep, and as I now do not think, I am not. This then is the consequence: A man not only falls asleep, but ceases to be when he falls asleep. There is no less fitting proof of the existence of the spirit of man than the theorem: ‘I think.’ Yet this began to be the most widely accepted statement in the age of evolution of Consciousness (the age of the Spiritual Soul). When we point to such things today, it is like a sacrilege — we cannot help ourselves!
Source: Rudolf Steiner – GA 237 – Karmic Relationships: Esoteric Studies – Volume III – Lecture I – Dornach, 1st July 1924
Translated by George Adams & D.S. Osmond