A well built theory has three merits: (i) it has an aesthetic appeal, (ii) it is comparatively easy to understand, and (iii), if its postulates are clearly stated, it may be taken out of its original physical context and applied in another.

The Hamiltonian Method and Its Application to Water Waves, Proceedings of the Irish Academy, Volume 63, Section A, Number 1, May 1962 (p. 1)