No one should make any personal inferences about themselves on the basis of whether or not I acknowledge them.

(a) I'm the stereotypical "absent-minded professor" and was that way decades before I become a professor. I get absorbed with the life I live in my head and become oblivious to what's going on around me. Sometimes people bring it to my attention, and I apologize. But obviously, these are only the people with enough self-confidence to ask, "whassup?" Others would go away feeling badly and possibly blaming themselves, instead of me.

(b) My eyesight isn't that good. But it's good enough so that I don't always wear my glasses. This means people who are more than about 6 feet away are not recognizable.

(c) My hearing isn't so good. I came across an article about mild hearing loss last week, and I realize that I would benefit from a hearing aid. But that's not gonna happen til I'm employed again. So my students will continue to put up with me saying, "Can you repeat that? My hearing isn't so good in my left ear."

In short, what is happening in the exterior world isn't always All About Me.