"I think, therefore I am". The famous phrase by Rene Descartes. Today I'll make a brief explanation of how he came to that absolute axiom and what can be derived from it.
If you want to discover a principle, doubt of anything that isn't 100% true all the time. The sensible world isn't certain, thus my senses aren't trustworthy. Anything I consider logic and true now can just be an illusion, a deceive. But, if it is indeed an illusion, there's necessarily someone who is being deceived. If the world is imaginary, there's somebody conducting cerebral processes and imagining the whole thing. Thus, If I think, I must exist.
Many things can be interpreted from this affirmation. What I take from this is that the most important thing is perception. What an individual thinks about something is more important than the object itself. (Of course this can't be applied all the time, or we wouldn't agree on anything.) The only thing other people know about you is what you appear to be.