From "Critique of Pure Reason" by I. Kant

"For we are unable from our own consciousness to determine whether, as souls, we are permanent or not. Since we reckon as belonging to our identical self only that of which we are conscious, we must necessarily judge that we are one and the same throughout the whole time of which we are conscious, We cannot, however, claim that this judgment would be valid from the standpoint of an outside observer. For since the only permanent appearance which we encounter in the soul is the representation 'I' that accompanies and connects them all, we are unable to prove that this 'I', a mere thought, may not be in the same state of flux as the other thoughts which, but means of it, are linked up with one another."