§153

The temptation - now that we've seen that the various 'surface' things that occur don't constitute the understanding - is to say that the understanding must be something beneath the surface. (In what sense are things 'on the surface' here? - It could be that they are on the surface in being observable behaviour or someone might think that they are on the surface in being 'available to consciousness' - something that we are aware of thinking).

The problem with this new temptation is that even if we did find something that happened in all cases what reason would we have to think that it was the understanding?
It must be that we already have criteria of understanding if we are able to correlate cases of understanding with something (e.g. a brain process).