If they provide information that you have never come across before. Lets say that someone told me about the mating habits of the shrimp at the volcanic vents on the seabed, I'm pretty sure I've never come across that information before. Thus the source of that information wasn't from me. Ergo, the sorce must exist outside of my imagination. It's like when a priest has been called in for an exorcism. If the subject is using a language unknown to them or has knowledge of events that have not transpired to them nor have information through another source then that person must be possessed!

You can't really, because everything IS Imagination. The outer world is an illusion. A fact, but not Truth.

Wow, I am having some difficulty trying to understand what youre asking - maybe cause I never took Philosophy classes in college... :P . The obvious one: youre asking me how can I prove that another person exists apart from me. How can I prove that this life, is in fact, not a dream or an illusion. If that's the question, I will simply say I can see, hear, touch, sometimes even smell and taste that other person. Because theyre in the realm of my sensory perception, I can confidently say they exist. . But I guess that cant be sufficient evidence of their existence. . I know the majority of human beings are born with these five senses as well - that makes it possible for me to compare experiences with them as well. And hearing their experiences tallying with mine or not tallying with mine, because they made different choices or because certain circumstances were different at that time, or because we're simply two different people shows me that the world works in a certain predictable way. . Using that world-works-in-a-certain-predictable-way-rule, if I meet someone who just doesnt seem to fit in... like maybe she shimmers and I could see through her, or maybe she's a full grown human but just ten inches tall - Id question if she were real, a hallucination or a spectre from another place/time :) . Okay who the heck is Descartes and why is he twisting my brain in knots like that for *googling Descartes*

A practical test would be to punch the person next to you on the nose and then I am sure you will feel their existence!

You can't really...and such is the plight of reality. Everyone has a different perspective. So, one could say that for each person there is a seperate reality. You have no way of knowing anything outside of your own reality, and thus cannot prove the existence of anything but yourself. It could be all a dream or, God forbid, it could just be the Matrix.

Unfortunately, those that argue that what you can touch, taste, see, hear, etc, don't realize the more complex part of Descartes argument that these are all constructs of an ultra-intricate ploy by your imagination. That being said, Hume completely shot down Descartes reality and stated that there isn't even a self. We are simply a bundle of perceptions that is not connected, so there is nothing you can call the self. So even I don't exist! However, Kant was the follow-up. He introduced his idea of the noumena, the faculties, intuition, the categorical imperative, and all sorts of other ideas that show that there is in fact a physical reality, but the one we observe is different from it. =)

OK, that's truly twisted. Wow. +4. Can't wait to see the answers on this one, but I really have to compliment you on a fascinating question -- I'm totally stumped. (And that ain't easy.)

i like philosophy.descartes tried to be the new socretes.IMHO they are great philosophers.emanuel kant says we cant know absolute reality or things in them selfs. it would be as having color glasses one lenses blue and the other red the image they project would be purple.we would think of reality been purple.

Wait until one of those "second individuals" comes up and slaps the dog-doo out of you. You'll know for certain they exist. : )

A tale Told by an idiot, full of sound and fury Signifying nothing.--Shak.[1913 Webster]"IS THIS ABSOLUTE REALITY"

It was actually Saint Augustine who first came up with the concept think/I am. Descartes simply took it, rearranged the wording, and expanded the concept. The real importance of "I think, therefore I am" coming from Descartes is that it was he, not Augustine, who formulated a theory giving the "mind" independent significance. This realization made it possible/easier for people to think of themselves as individual thinkers, rather than being connected to some ultimate soul, or something. Not that Descartes was an atheist, because he certainly was not. But he attempted a methodology in hope that we as individuals can discover truth and certainty for ourselves, and be confident in that knowledge. We don't have to be dependent on others (the Church, for ex. in his age) for knowledge. We can think for ourselves!