As scientists, we like to think our theories are built on a large body of directly observed phenomena, but we can directly observe only a few, primitive things, like the relative position of two objects -- and even that is called into question by modern physics! Instruments that measure abstract quantities like force and temperature depend on the validity of an underlying theory. For example, a scale provides a useful measurement of weight only if we accept a certain definition of weight. Such definitions are objectively arbitrary, because they can only be ranked in terms of usefulness.