A general theory of inertia tends to be circular because momentum and therefore inertia are taken as assumptions in quantum field theories. In this paper we explore instead using position uncertainty to infer inertia with mediation by quasi-measurement interactions. This method avoids attachment to the reference frame of the source masses and is thus completely relativistic, overcoming a conflict between historical theories of inertia and relativity. We investigate what laws of motion result, and whether natural explanations for equivalence and dark energy emerge.