The analogy that he draws is with conservation laws such as the conservation of energy. This is not a law of physics like quantum mechanics or relativity but a principle that all other laws must follow. As energy is converted from chemical to electrical to kinetic to potential energy and so on, its behaviour is governed by all kinds of different laws of physics. But they must all obey the principle that energy is conserved.

“Thus the conservation law, though not an a priori mathematical truth, provides an explanation of aspects of motion that is deeper than laws of motion,” say Deutsch.

This approach solves a number of problems. In particular, information has always been difficult to define. In conventional information theory, information and distinguishability are each defined in terms of the other, creating a kind of chicken and egg problem.

But in constructor theory, th...