The author considers it a mistake to look on defense and anxiety as functions exclusively of the ego, thereby denying their instinctual quality. He proposes a revision of instincts into two groups: the erotic or vital-libidinal instincts such as feeding and digestion and the several components of the sexual instincts, and the defensive and aggressive instincts. A physiological concept of primary and secondary instinctual sources of stimulation for both sets of instincts is outlined and illustrated. It is suggested that this physiological substratum of instinctual drives, whether these be feeding or respiration or defensive-aggressive drives, is of great importance for analytic theory.