This article is dedicated to the philosophy ofscience which was developed by the outstanding Soviet physicist and leader of a powerful scientificcommunity, L. I. Mandelstam. It is shown that thisphilosophy can be summed up under the heading “operationalism”. A comparison with the paradigmaticoperationalism of Percy Bridgman is undertaken andthe German positivist roots of Mandelstam's philosophyare indicated. The final section reconstructs the principle ofexpedient idealization, the principle which was putforward by Mandelstam's disciples in the spirit of hisoperationalism to solve problems of the theory ofnon-linear oscillations.