ABSTRACT
Descartes was doubting everything, to see if any belief was quite impregnable. The Cogito seemed such a one, and others might later be welcomed back by the same criterion. In stating this line of thought he did not always trouble to distinguish the inference itself from the principle which governed it: that thoughts need thinkers. The other premiss, ‘I think’, seems guaranteed by being so general and being said about oneself; but it cannot be relied on until the conclusion is reliable, which (if we are relying on that inference) means relying on those premisses. The argument goes round and back again to its starting-point, and can never prove “I think”.
