“You began by speaking —should I say again to the philosopher— of the brain such as we see it, such as it stands out in the midst of the presentation: so you assumed it to be a part of presentation, an idea, and you were in idealism. There, I repeat, the relation of the brain to the rest of presentation can only be the relation of part to whole. Thence, all of a sudden, you have fled to a reality supposed to lie beneath the presentation. Very good: but such reality is subspatial, which amounts to saying that the brain is no more an independent entity. What you have to do with now is the totality of the real, in itself unknowable, over which is spread the totality of the presentation. You are now, indeed, in realism; and no more in this realism than in the idealism of a moment ago are the cerebral states the equivalent of the whole of presentation: it is – I must repeat it – the whole world of things which is again implied (but, this time, concealed and unknowable) in the whole of perception. But lo! taking the brain apart and dealing with things separately, you are actually continuing to decompose and recompose reality along the same lines and according to the same laws as presentation, which means that you no longer distinguish the one from the other. Back you are, then, in idealism; there you ought to remain. But not at all! You do indeed preserve the brain as it is given in presentation, therefore as an idea, but you forget that if the real is thus spread out in the presentation, if it is extension and not tension, it can no longer compress within itself the powers and virtualities postulated by realism; unheedingly you erect the cerebral movements into the equivalent of the whole of presentation. You are therefore oscillating from idealism to realism and from realism to idealism, but so quickly that you do not perceive the see-saw motion and you think yourself all the time astride the two systems joined into one. This apparent reconciliation of two irreconcilable affirmations is the very essence of the thesis of parallelism.”

Henri Bergson