Einstein made a distinction between principle theories like Newtonian mechanics and constructive theories like kinetic theory of gases. Are these two distinct types of theories fundamentally different from each other or can they be regarded to belong to just one type of theory? We explore this issue with respect to the theory of scientific study and come to the conclusion that there is only one type of (scientific) theory, and the constructive theory is a principle theory with only one principle, (...) which we call the default-principle theory rather than calling it a constructive theory. One reason why constructive theories are considered as default-principle theories is that it provides a natural progression from default-principle theory to a principle theory as science progresses. This also avoids the suggestion that constructive and principle theories are considered as completely distinct entities without any interaction with each other, which may hinder scientific progress. (shrink)