Technically I am just a process, a journey the philosopher may say.
Nothing can contain me.
For men may come and men may go, I go on forever1
Take what you want or give what you want I remain just the same.
I have an opposite and it's not death.
What does this make me?
1 (Taken from the poem "The Brook")
Edit:- Edited the question to reduce the number of possible answers. Please let me know if I should edit anything else.