In the physical sciences, some very basic facts or principles appear to have a status that is difficult to categorize: not simply empirically discovered; not purely analytic (true by virtue of already established meanings); fundamental, but without quite being ordinary physical laws. Incompatible-looking alternative principles are conceivable; sometimes we can even see how an alternative physical framework could be built on them. Such principles are held, by some philosophers, to be true by convention. They are parts of our physical theories that had to be conventionally chosen by us over other incompatible postulates, whether or not we were overtly aware of this element of choice.

The most famous examples of putative conventional truths are to be found in our theories of space and time, and will be discussed below. But some are not directly related to space and/or time: in classical physics, Isaac Newton's famous 2nd law...