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There is a popular intuition that mental events are different in some important way from physical events. Yet we are taught to consider everything as belonging to a physical world. There are a number of different ways to try and resolve this apparent paradox, relating to a variety of implications of the word mental, not all of which necessarily go together all the time. There is, however, one simple way of explaining why mental and physical events seem different, which relates to the way the brain works.
A physical event is something we can allocate both temporal and spatial extent because we have ways of calibrating the world in terms of distance, angle and time. I can register the flight of a bird from one tree to another as an event with shape and duration because my nervous system has learnt to calibrate the world in terms of binocular disparity in my right and left visual fields, changes in appearance when I walk around or turn my head and even, for an bird at night, subtle changes in the phase, pitch and reverberation of sounds reaching both ears. Interestingly, there are physical experiences we are happy to observe without all of the normal features. A distant firework display has no knowable size or depth, just a two dimensional pattern. Nevertheless, physical events usually have size shape and duration.
If we now turn to a mental event, such as my thinking about cooking some rice an hour ago, it has a duration, much as the cooking of the rice did, but although it may seem to have a location in the vicinity of my body, it has no size or shape. Mental events seem to have no spatial structure. This might be interpreted as indicating that they do not in fact have any spatial structure. Alternatively, it seems reasonable to suggest that we simply have no means of determining the shape or structure because we have no calibration tools.
This latter possibility makes a lot of sense because nothing in the brain moves in relation to anything else. In order to have memory we must have parts of the brain that monitor the sequences of their input signals and store patterns that allow those sequences to be replayed and attributed times and durations. But nothing in the brain can monitor where anything else in the brain is because there is no way of calibrating distances without movement. Nothing in a brain can have any idea that events are occurring in the sorts of spatial patterns we see on fMRI scans. In other words we have internal clocks but no internal rulers and for that reason alone mental events can be known in terms of durations but not size or shape.
Arguably, mental and physical events are otherwise pretty much the same. We might say that loudness and redness are aspects of physical events whereas nausea and pain are aspects of mental events but again the difference seems to be fairly easily related to availability of calibration against other things.
