I would like to learn these, and many other things about image processing, from a scientific point of view. I want to learn facts, not magic recipes.
Edit: Just read through the thread properly(!) so here's my shortened response:
PP is just about getting a finished image that's a decent match for the mental vision you had when you took the photo. That's the driving force; when you don't have the PP knowledge/technique or tool to produce what you want, then you learn/buy and expand your repertoire. There's no need to use Photoshop if you can get what you want without it.
Personally I think a good way to get a handle on this is to join a forum that has an active "PP This" type of thread - a thread where someone posts up an image and other members each have a go at processing it, describing what what they did along the way, and why. Here's one I'd recommend, in that it generally avoids the more adventurous edits (such as background/subject replacements) and has a generally helpful atmosphere:http://www.talkphotography.co.uk/threads/the-pp-game.347318/
You'll see lots of different "takes" on the same image. Which one is best? How long is that piece of string?