§158

Do we say that we cannot determine the first word read by looking at a physical mechanism or process (say, in the brain) because we don't know what is going on in the brain? - Perhaps if we did know then the brain mechanism would be the criterion.
Wittgenstein rejects this. Those who think that it must be that way (that there must be a brain mechanism that would allow us to determine the first word read) have a certain a priori model. Those who think that way are held captive by a picture - an appealing form of representation. - They are conceptually confused.