The object of this course of seminar lectures is, in Professor Schrodinger's words, to develop briefly one simple unified standard method, capable of dealing, without changing the fundamental attitude, with all cases (classical, quantum, Bose-Einstein, Fermi-Dirac, etc.) and with every new problem that may arise. The interest is focused on the general procedure, and examples are dealt with as illustrations thereof. It is not a first introduction for newcomers to the subject, but rather a 'repetitorium'. The treatment of those topics, which are to be found in every one of a hundred textbooks, is severely condensed; on the other hand, vital points which are usually passed over in all but the large monographs are dealt with at greater length.