In Science-speak we have ‘postulates’ and ‘axioms’. 

These terms are interchangeable.  They both refer to the same ‘assumption’ and the underlying philosophical logic which supports that assumption.  Creating a ‘postulate’ or an ‘axiom’ was common with ancient Greek philosophers and the medieval schoolmen and naturalists.  Neither a postulate, nor an axiom is by itself a ‘truth’.  It is not by itself a ‘law’.  It is simply an assumption and is used as a ‘starting point’.

·        Moons orbit Jupiter.

·        Our moon appears to orbit the Earth.

·        Therefore, the Earth must orbit the Sun, given the similarities in this relationship that we see with moons and their planets.

·        My theory will start with the ‘postulate’ or ‘axiom’ that the Earth orbits the Sun. 

·        Based on this ‘axiom’ I will declare that the Sun and our solar system must revolve around the centre of our galaxy.

·        I will now expand on this hypothesis with my proofs and maths….

The above ‘postulate’ uses an assumption that is assumed to be a truth.  I have just accepted that premise and moved on.  This is not exactly scientific.  A critic could and should go back to my foundational ‘axiom’ or ‘postulate’ and ask for experimental proof.  If the axiom is so obvious, that proof should be readily available.  More here