A First Principle can be seen as a Prime Directive (as a modern pop culture reference). It is basically determined by you. You choose what is.
However, there’s a little bit more to it;
… in the first place, they must be so clear and evident that the human mind, when it attentively considers them, cannot doubt of their truth; in the second place, the knowledge of other things must be so dependent on them as that though the principles themselves may indeed be known apart from what depends on them, the latter cannot nevertheless be known apart from the former.
