The following notes are by Wittgenstein, from Zettel.

605. One of the most dangerous ideas for a philosopher is, oddly enough, that we think with our heads or in our heads.

606. The idea of thinking as a process in the head, in a completely enclosed space, gives him something occult.

607. Is thinking a specific organic process of the mind, so to speak — as it were chewing and digesting in the mind? Can we replace it by an inorganic process that fulfils the same end, as it were use a prosthetic apparatus for thinking? How should be have to imagine a prosthetic organ of thought?

608. No supposition seems to me more natural than that there is no process in the brain correlated with associating or with thinking; so that it would be impossible to read off thought-processes from brain-processes. I mean this: if I talk or write there is, I assume, a system of impulses going out from my brain and correlated with my spoken or written thoughts. But why should the system continue further in the direction of the centre? Why should this order not proceed, so to speak, out of chaos? The case would be like the following — certain kinds of plants multiply by seed, so that a seed always produces a plant of the same kind as that from which it was produced — but nothing in the seed corresponds to the plant which comes from it; so that it is impossible to infer the properties or structure of the plant from those of the seed that comes out of it — this can only be done from the history of the seed. So an organism might come into being even out of something quite amorphous, as it were causelessly; and there is no reason why this should not really hold for our thoughts, and hence for our talking and writing.

609. It is thus perfectly possible that certain psychological phenomena cannot be investigated physiologically, because physiologically nothing corresponds to them.

610. I saw this man years ago: now I have seen him again, I recognise him, I remember his name. And why does there have to be a cause of this remembering in my nervous system? Why must something or other, whatever it may be, be stored up there in any form? Why must a trace have been left behind? Why should there not be a psychological regularity to which no physiological regularity corresponds? If this upsets our concept of causality then it is high time it was upset.

611. The prejudice in favour of psychophysical parallelism is a fruit of primitive interpretations of our concepts. For if one allows a causality between psychological phenomana which is not mediated physiologically, one thinks one is professing belief in a gaseous mental entity.

612. Imagine the following phenomenon. If I want someone to take note of a text that I recite to him, so that he can repeat it to me later, I have to give him paper and pencil; while I am speaking he makes lines, marks, on the paper; if he has to reproduce the text later he follows those marks with his eyes and recites the text. But I assume that what he has jotted down is not writing, it is not connected by rules with the words of the text; yet without these jottings he is unable to reproduce the text; and if anything in it is altered, if part of it is destroyed, he sticks in his ‘reading’ or recites the text uncertainly or carelessly, or cannot find the words at all. — This can be imagined! — What I called jottings would not be a rendering of the text, not so to speak a translation with another symbolism. The text would not be stored up in the jottings. And why should it be stored up in our nervous system?

(Anscombe, G.E.M. and Von Wright, G. H. (eds.) (1967). Zettel. pp.105–7)

Sean B. Palmer