 Although Ludwig’s meta-theoretical framework is slightly different, the first part of his solution is essentially equivalent to the above one. On the other hand, he proposes a stronger program (“axiomatic basis of a physical theory”) which proceeds by considering an equivalent form \(T\)* of a theory \(T\) in which all \(T\)-theoretical concepts are eliminated by explicit definitions. This seems to be at variance with older results about the non-definability of theoretical terms, but a closer inspection removes the apparent contradiction. For example, the concept of “mass” may be non-definable in a theory dealing only with single orbits of a mechanical system, but definable in a theory containing all possible orbits of that system. 