FallenTraveler said:
for aircraft sims it should be inverted though, and for quite a few other things it should be as well, but for fps's it just doesnt make logical sense to invert it... at least to me.
On the other hand, one could argue that it doesn't make sense to keep switching based on what game you're playing, and instead makes more sense to pick one scheme and apply it to everything.
The first game I ever played with view controls was a flight sim, so it's become ingrained for me to invert most everything. A few years back I got into a discussion with my sister while playing Kingdom Hearts about the X axis inversion. She was utterly confused as to why I would prefer to invert anything, saying that it made more sense to just push the control stick in the direction you wanted to look. I think explanation I gave her is applicable here, as well.
To me, I usually think of the camera system in a game as a class 1 lever. The camera is the force, the player character (PC) is the fulcrum, and your view is the load. To move your view (the load) you have to move the camera (the force) in the opposite direction, and they will pivot around the fulcrum (PC.) Here's a crappy image to illustrate. Imagine this is a top-down view.
X = Camera, Y = PC, Z = field of view
To swing your view to the character's left, you move the camera (control stick) to the right. The camera will swing around and be looking to the character's left.
You could also imagine that this is a side view, and the player is pressing back on a control stick or pulling back on a mouse to pan the view upwards. In 1st person games, X and Y are the same entity, but some people still use this logic then. It's just preference.
3rd person = inverted X and Y axis
1st person = inverted Y axis, natural X axis