A defining question

[TomOfSweden]

I live life with the idea that we cannot truly prove anything, and so anything can be true to anyone... because I may not view reality the same way as someone else.

I just want to point out here that there are no philosophers, (since the Sophists in ancient Greece) who have held this view to be correct.

I'm not sure if this is what you're doing, but this reminds me very much of what I hear in on-line philosophy discussions, and is a very common misunderstanding of the post-modern project. No matter how relative these philosophers, (Derrida, Foucault, Lacan, Zizek, Heidegger, Nietzsche, etc) think it all is, they all still believe that there is only one truth out there, no matter how you look at it. Philosophical relativism means asking the question, "what is this relative to" and rejecting the idea that anybody has an elevated position from where they can see everything clearly, (like Hegel and Marx thought they had). This is not the same thing as "anything can be true to anyone". I suspect that at its at its core its mostly just intellectual laziness. Philosophical relativism doesn't make anything easier to understand. It adds lots more layers of complexity.

Could it be that what you are proposing is simply Solipsism? Which no philosopher ever has held to be true.

In a murderer's reality they believe it is true that killing is good... and whilst I don't share this truth, and certainly don't respect it... I have to acknowledge that unless I can somehow make this person see reality the way I do, their truth is still true to them.

A person who is wrong but believes they are right, is still wrong. You know that their truth, is based on faulty premises. How can their truth be right even to them? Aren't you simply opening up the possibility that both of you could be wrong? Nothing wrong with that, but there's still only one truth.

Perhaps the problem with defining truth is that, in order to define it, we have to establish a truth... we cannot know what we percieve to be truth is true.

This I think is confusing. No matter what we perceive, there's still only one truth. There's plenty of pre-prepared systems to apply and then just walk through the steps.
http://en.wikipedia.org/wiki/Truth
That's what's so handy with philosophy. Somebody else has most often done all the heavy lifting for you.

I'm loving "we can't figure out what's true anyway, so who cares." though... very accurate... well... to me anyway

The goal with pragmatists was to go beyond these basic quandaries, without just picking one of the solutions and then glossing over it's problems. Like so many philosophers before them had done.