Mitch-
This is an interesting question, the sort of thing philosophers like to argue about.
You
might ask your friend why he values seeing so much more than the other
senses. If he were to go blind in old age, would he think that the
world had disappeared or that he had lost use of one of his senses? You
can certainly FEEL air when it blows on you.
Also, you can
indirectly see the effects of air. For example if you watch some smoke
rise, you can see it shift direction when you blow air on it with a
fan.
You can also use a pump to remove most of the air from a
container. If the container isn't strong, it will collapse. That's from
the air pushing in on it, with not much air pushing back out. You can
even weigh a conatiner with the air pumped out or not, and find a small
difference. You can even see air. By seeing something, we mean shining
light on it and looking at the light that bounces off of the object. We
see the sky as blue because blue light bounces off of air molecules
more than red light does. Please see our
answer to the blue-sky question.
One interesting thing you can do with air is you can get it so cold
parts of it or all of it will turn into a liquid or solidify. The
different components will liquify or solidify at different
temperatures. The water vapor is first to go, condensing at the dew
point and making frost at freezing temperatures. Carbon dioxide will
make dry ice at lower temperatures. Oxygen and nitrogen liquify at
cryogenic temperatures. Air is composed mostly of nitrogen. The physics
van does lots of
demonstrations
with liquified nitrogen, which can be used to cool other things. Liquid
nitrogen is certainly plainly visible -- it looks like clear water.
Certainly your one's belief in the existence of a substance doesn't
depend on its temperature? There are many, many other ways to convince
yourself that air is as real as anything else. Vision is a great sense,
but it's only part of how we sense the world.
Mike W. (and a little bit from Tom J.)
(published on 10/22/2007)
