The first postulate is a truism.

Perplexity:
"The First Postulate of Special Relativity
Statement of the First Postulate
The first postulate of special relativity, also known as the principle
of relativity, states:
    The laws of physics are the same in all inertial frames of
reference."
"truism
/ˈtrˌizƏm/  n. a statement that is obviously true and says nothing new
or interesting. —truistic/trˈistik/ adj." -Oxford American.