@freemo it's a long explanation (considering the course at university was 4 months long) but basically it's an assumed contract that all humans agree to. It's what our definitions of right and wrong is based on. You are unknowingly taught it through the education system, through morals and customs.
The basic premise is that the freedom of one's right to choice is only valid so long as it does not infringe upon another's right to choice.
If you want the full unabridged version though...