Einstein's first researches in physics (particularly those of the golden year 1905) delt with two domains which he treated independently: the atomic and radiative domain on one side, electrodynamics on the other side. Later on, these two directions would cristallize, the first one towards quantum theory, the second one towards the theories of relativity and of the continuous matter field. When looking at Einstein's works in both directions (which he always maintained in parallel), one sees already at the beginning two different ways of approach: a heuristic one, making use of a probabilistic method of investigation raised up from thermodynamics (calculation of fluctuations), with the aim of characterizing specific properties of the new domain of quantum physics that was breaking with known theories and concepts; and another one, decidedly fundamental, organizing the theoretical work around sound physical principles, in particular invariance principles. These two modes of approach do not show a splitting between two different attitudes of thought, an empiricist one on one side, and a theoretical-rational one on the other side (as many commentators believed, referring them to two distinct periods of Einstein's achievements), but to a differenciated manner, proper to him, of conceiving the theoretical work, in function of the possible intelligibility of its object, in terms of concepts and principles that were always thought physically. This manner defines Einstein's own style as a searcher in physics, at the same time critical and constructive, a style which was being constituted since his first works.
Completeness; Einstein; Fundamental theory; Heuristic view; Mathematical form; Physical concepts and principles; Quantum physics; Scientific style