Pick any location in space — any location at all
The location can be completely arbitrary; you are free to put it anywhere. For the sake of this discussion, call this location ‘ansin‘. Ansin can have two spatial properties relative to each of my eyes:
(a) The spatial distance between each eye and ansin.
(b) The direction of ansin relative to each eye.
How do we determine ‘b’ — the direction of ansin relative to each eye? Is Ansin the same distance and direction from each eye?
First, yes, ansin is trivially the same distance and direction from each eye — if each eye is in the same place as the other eye. However, each eye is not in the same place as the other.
Second, yes, ansin is at least practically the same distance and direction — if the distance between ansin and each eye is so far that the differences in their locations serve no practical purpose.
For example, say ansin is the Andromeda Galaxy, or some point within it. Say my eyes are a few inches apart, both point the same way, and I am looking sideways at Andromeda. Strictly speaking, ansin lies at a different distance and direction to each eye, but it’s a tiny tiny difference.
It doesn’t work so well, though, if ansin’s distance is comparable in scale to the distance between the eyes. For example, if it’s a dust mote floating by my left eye.
Nor does it work if my eyes point in different directions. If I look cross-eyed at the sky, and one eye faces ansin in Andromeda directly, even if it looks to the same distance, my other eye is likely looking at a very very different point in space — one light years away from Andromeda. Differences in direction increase differences in position the further out you go.
You can probably tell a similar story about ears, and hands, and parts of the skin, bits of the nose, areas of the tongue, nerves in the body and whatever else you hold to be the set of organs related to (a) any particular sense and/or (b) all the senses together.
I think that this fact –that the organs of each sense (and all of them together) are distributed in space — matters. It matters when we come to saying that there is some error in the resulting perceptual experience. In particular, it matters when we come to classify any such error as illusion or hallucination or even as an error at all. I think the following is always a possibility: we interpret the experience as mistaken because we do not consider some limit to the experience itself. Our decision is influenced by how we understand the experience to be related to its sense-organs, and how we understand these organs to be distributed in space.[Note 3]