The first problem is the problem of when an observation is made. If we believe that every set of events at one instant is entirely separate from the set of events at the next instant (presentism) we can only know the immediate past from reports in the form of memories and these reports could be wrong. If this cosmology were correct then there would indeed be grounds for doubting all reports about experience although no grounds at all for dismissing all reports. This all sounds reasonable until you actually inspect your experience. When you actually observe what is happening you have whole words and movements in experience so experience is time-extended and presentism is false. There are not even grounds for doubting the events in your immediate experience, let alone discounting them, unless you are a presentist.
Notice that whilst you are continuously viewing a word on this screen there is no time for any process to intervene between your perception and itself. What you are experiencing is here, now. However you might be surprised that what is here now is not simple - see The form of objects (also see the discussion of simultaneity below).
The second problem when interpreting experience is the level of measurement available within the experience. Can I apply nominal, ordinal, interval or ratio measurements to experience? I will explain these terms below and consider which levels of measurement can be applied.
When I observe an object such as this screen it continues in the extended present of my experience and is separate from other objects. This possibility of separation and classification means that my experience is consistent with a nominal level of measurement.
As well as separate events my experience has arrangements of these events. Indeed, if there were no arrangements then there would be no separate objects because objects are fixed arrangements of events. Arrangements of events are consistent with an ordinal level of measurement.
The existence of ordinal scales in experience means that experience may have independent axes for organising these arrangements. This is clear from the possibility of experience itself because experience would not exist if it were not more than a single point. Extra points are arranged around a single point in experience to make a surface so two dimensional arrangements can be assumed to exist within experience. In other articles on this site it is argued that experience may contain at least four dimensions (including time).
We cannot place a measuring rod into our dreams so it is difficult to maintain that we have interval or ratio measurements available in experience. However events separated by ratio or interval scales may exist but be impossible to measure directly. This is a highly important point: the lack of ratio or interval measurement in some parts of experience does not mean that ratio scales cannot be applied to some events in experience.
If I make a statement such as "I imagine the orange is twice the size of the apple" the imaginary orange may indeed subtend twice the angle at my viewing point as the imaginary apple but I cannot measure this ratio. Should I abandon any hope of a theory about the imaginary fruit? Obviously not. Physicists are confronted by this problem every day, no physicist has ever seen an electron or an atom, no physicist has ever stuck a measuring rod across an electron, instead the physicist develops a theory based on the assumption that the electron could be represented on a ratio scale and uses this theory to predict the ratio measurements that can actually be made.
When a person says that one imaginary object is twice the size of another it is an hypothesis, not an impossibility.
The third problem when interpreting the form and content of experience is the problem of location. Someone suffering from hallucinations may claim that there are pink elephants on their wall but these will be invisible to other observers. The person who sees the pink elephants may be able to measure them but then where is the 90cm high pink elephant? There is a meditation on the location of events in experience at: The spatial modes of experience. The answer appears to be that the pink elephant is in the space of their experience, a possibility that is obvious if indirect realism is accepted.
|Muller Lyer Illusion|
It is also obvious that there can be no three dimensional visual homotopy because vision is not three dimensional - we do not have the rear of objects in our experience. The problem of visual homotopy applies narrowly to the correspondence between the projected image of the world around us, such as the image created by a lens, and the projected image within visual experience.
The flexible positioning of events in perception that underlies the Muller Lyer illusion is also seen in our adaptation to wearing glasses. If I walk around wearing cheap reading glasses for several hours and then take them off to view a window frame I can see the edges of the frame bowed out rather than upright. If I replace the glasses the edges become upright and if I leave the glasses off for a few hours the edges return to upright. The form of my perception is actively returned to a preferred form.
The preferred layout of my perception could be continuously isotropic or anisotropic. (Lengths could be preserved from place to place within perception or vary). Perceptual space could not be generally discontinuous because then as objects move or rotate it would be evident that parts would disappear (eg: the blind spot is a discontinuity in the world that loses data). Given that the layout of perception can be actively changed and homotopy is the simplest layout it might be proposed that perceptual space is continuous and approximately homotopic with the space that it represents: the relations between events in perception are actively changed to an approximate homotopy between perception and images of the world. The word "approximate" is important because the blind spot and optical illusions show that perceptual space is definitely not perfectly homotopic with the layout of objects in the space beyond the body.
The homotopy is never perfect but given that active homotopy exists it is reasonable to advance hypotheses that deal with events in perception at a ratio level of measurement. The proof of such hypotheses would depend on how well they account for the relations within experience and the relations between experience and the world and brain.
This brings us to the possibility of homotopy between perception and events in the brain. Close inspection of perception (see Meditations) suggests that what we believe to be spatial arrangements of events, most obviously along the radial direction in visual perception, can actually be temporal arrangements so what is needed is a deeper understanding of the empirical form of experience before perceptual-neural homotopy can be explored.
A fifth problem is the problem of simultaneity in experience. This is not the problem of whether what is simultaneous in experience is simultaneous in the world, it is the problem of whether anything in experience is simultaneous. If we stare at a uniformly coloured area of space the colours form a surface and we conventionally believe that this surface is simultaneous. There have been no successful attempts to attack this convention, and where there have been attacks on the simultaneity of experience, for instance Dennett and Kinsbourne (1992), these have been attacks on the perceived ordering of events, not on whether or not simultaneity actually exists within each event or object. For example, Dennett and Kinsbourne talk of the ordering of "lights" but these lights are objects that are simultaneously coloured.
When I look at a uniform green field it is clear that the hundreds of thousands of photoreceptors cannot be contributing to perception one at a time because it would take too long to see anything. This means that the contention that we have no simultaneity in experience would be equivalent to a dualism in which experience occurs somewhere outside of space and time. Even if this were the case there would need to be some common property that makes a blue light or blue object "blue" in all its parts so this particular argument against simultaneity would amount to an argument that there is a mental time that is not the same as physical time.