Einstein on the difference between construct and principled thinking...

"We can distinguish various kinds of theories in physics. Most of them are constructive. They attempt to build up a picture of the more complex >phenomena out of the materials of a relatively simple formal scheme from which they start out. Thus the kinetic theory of gases seeks to reduce >mechanical, thermal, and diffusional processes to movements of molecules -- i.e., to build them up out of the hypothesis of molecular motion. When we >say that we have succeeded in understanding a group of natural processes, we invariably mean that a constructive theory has been found which covers >the processes in question.
Along with this most important class of theories there exists a second, which I will call "principle-theories." These employ the analytic, not the synthetic, >method. The elements which form their basis and starting-point are not hypothetically constructed but empirically discovered ones, general characteristics >of natural processes, principles that give rise to mathematically formulated criteria which the separate processes or the theoretical representations of >them have to satisfy. Thus the science of thermodynamics seeks by analytical means to deduce necessary conditions, which separate events have to >satisfy, from the universally experienced fact that perpetual motion is impossible.
The advantages of the constructive theory are completeness, adaptability, and clearness, those of the principle theory are logical perfection and security >of the foundations. The theory of relativity belongs to the latter class."