No one can see me, everyone hears me, I converse with you eternally. I speak every language, I make all the new ones, nothing but death surpasses me. I am the mastermind of criminals, as well as the heroes, I know all things anything has discovered. I don't take form, I only exist, and if you ask me, I only give you your best guess. Who or what am I?
Conscience is an internal voice, so everyone hears, but noone sees. As everyone has one, all languages are represented. Death kills the inner voice.
No there yet:
"I know all things anything has discovered" - not sure about that one.