Friday, June 26, 2009

Dark nebulae

Last night, I was observing a portion of the Pipe Nebula, which is a dark lane of dust obscuring the Milky Way. It was kind of cool: I could see stars to the left of it through the telescope, and then as I moved the telescope to the right, the field of view went almost completely dark, except for some stars on the fringes and some quite faint stars in the middle. But did I see the Pipe Nebula? It seems that what constituted "seeing" the Pipe Nebula was my not seeing the stars behind it. After all, intuitively seeing seems to be a causal process whereby the seen object causes light to reach the eyes. But the Pipe Nebula did not cause any light to reach the eyes. Of course, the same issue comes up when one "sees" a matte-black cube, a shadow, etc.

Maybe, then, we need to relax the intuitive concept of seeing as a process whereby the seen object causes light to reach the eyes. One might say that seeing is a process whereby the seen object causes light to reach or not reach the eyes (or, maybe better, causes a particular profile of wavelengths of light to reach the eyes, which profile might be empty). But if that were right, then we should say that a blindfolded person sees the blindfold (and that a person with eyes closed sees the inside of the eyelids). However, I do not think we say that—we say, rather, that a blindfolded person does not see anything.

Here is an alternative that accounts for shadows, ultra flat black cubes and dark nebulae: in seeing, the seen object causes a non-empty shaped pattern of light. There are two ways of causing a shaped pattern of light: one way is by causing the light (by reflection, emission or refraction) and the other way is by causing the shape. Here, "shape" must be understood in such a way that a field of view filled with uniform light counts as a shape (so that if one is right up against a uniform blue wall, one still sees the wall) while an empty field of view, as in the blindfold case, does not count as a shape.

A problem with this account is that if one's face is up against a red, or green, or blue wall, one counts as seeing the wall, but if the wall is painted with ultra flat black paint, then one isn't seeing the wall. I do not know how to give an account on which one counts as seeing the ultra flat black wall, but one doesn't count as seeing the blindfold.


Alexander R Pruss said...

A complication is that sometimes one can "see" a dark shape by panning. Thus, if one's face is right up against a big black disc on a white wall, and one moves one's face (still against the wall) to the edge of the disc so that one sees the whole outline, then one counts as having seen the big black disc. And then when one's face is in the middle of the disc, so one can't see the edges, one still count as actively seeing the black disc. On the other hand, if one's face is pressed against the black disc, but seeing the edges of the disc is not a part of this perceptual session, then one counts as not seeing anything, I think.

This show that whether one counts as "seeing" something is going to have some interesting borderline cases.

Chris said...

Clever puzzles. I especially like the face-against-the-black-wall scenario. Let me give what I think is a roughly Reidian answer to each case.

For a perceiver to have a perception of an object, the object must be causally related to a sensation had by the perceiver. The sensation, in turn, must trigger an immediate (i.e., without inference) conception of the object, and the perceiver believe in the object’s existence.

In the case of the Pipe Nebula, it seems to me that the nebula is causally related to the pattern of light projected onto your eye, and that pattern determines the sensation you have. I don’t know whether you immediately had the conception, “the Pipe Nebula” or “whatever is blocking the Milky Way” or any other conception of the Pipe Nebula, or you made some inferences in the process. Let’s suppose the former. Obviously, you eventually had the conception and belief in its existence. Then, it seems to me that you did perceive the nebula. And, because you perceived it visually, it is correct to say that you saw it.

It seems that this helps with the blindfold case. If one opens one’s eyes with a blindfold on, one has a uniform black sensation. But such a sensation might be caused by any number of things—blindfolds, lights being off, low blood pressure, etc. It would be very strange if the simple black sensation triggered “blindfold.” Rather, this conception comes from the tactile sensation of having the blindfold on one’s head. It would not be correct to say that one sees the blindfold, but one does feel it. On the other hand, if someone had sufficient experience with blindfolds and little experience with other dark conditions, one might develop a tendency to think “blindfold” anytime one’s visual field goes black. Such a person would seem to have acquired the ability to see one’s own blindfold. The flat black wall seems similar. Except in very unusual cases, one’s visual sensations (or lack thereof) would not immediately cause the required conception or belief for a perception.

In the case of the disc or the colored walls, even if one does not have the conception, “disc” or “colored wall”, one could have a relative conception like “whatever is causing that red sensation”. So, these cases look like they would be cases of seeing.

This seems to fit your intuitions. One problem for this view might go something like this: Suppose a large bell rings. The auditory sensation causes a perceiver to conceptualize and believe in the bell. Curious about what may have struck the bell, the perceiver turns to look at the bell. She has a visual sensation caused by the bell. But she already had the conceptualization and belief in the bell. But surely she sees it.