Well, I have a privileged position of being able to derive it from the first principles, so it is “true” given certain rather mild assumptions about the way the universe works, which stem from some observations (speed of light is constant, observations leading to the Maxwell equations, etc.) leading to the relativistic free particle Lagrangian, and confirmed by others (e.g. atmospheric cosmic ray muon decay). So this is not an isolated belief, but more like an essential part of the model of the world. Without it the whole ontology falls apart. And so does epistemology.
Well, I have a privileged position of being able to derive it from the first principles, so it is “true” given certain rather mild assumptions about the way the universe works, which stem from some observations (speed of light is constant, observations leading to the Maxwell equations, etc.) leading to the relativistic free particle Lagrangian, and confirmed by others (e.g. atmospheric cosmic ray muon decay). So this is not an isolated belief, but more like an essential part of the model of the world. Without it the whole ontology falls apart. And so does epistemology.