Abstract
This article explores Einstein's views on the nature of scientific theory, and is directed towards students and researchers in the physical sciences and engineering. A majority of scientific theories belong to the constructive type whose axiomatic foundation consists of empirically observed principles and imaginary or speculative hypotheses. On this base is erected a logical superstructure (e.g. calculus) whose object is to make concrete predictions which can be tested against experiential data. A favourable comparison will point to validity of the hypotheses while an unfavourable one will require their revision. A constructive theory holds temporarily and will eventually be replaced with a more comprehensive approach. In contrast, in a theory of principle, the axiomatic foundation consists solely of principles of nature that are derived from human experience. Unless these principles are found to be false in the future, the security of such a theory is guaranteed. An example of a constructive theory is the kinetic theory of gases while classical thermodynamics and the theory of relativity belong to the class of principle theories.