Pavlov's conceptualization of unconditional reflexes, or instincts, within the framework of the theory of higher nervous activity.

According to I. P. Pavlov's theory of higher nervous activity, the establishment and dissolution of conditional reflexes enhances the higher organism's adaptation to the external environment. Pavlov asserted that, ontogenetically, conditional reflexes are based upon innate, unconditional reflexes (UR) or instincts. Pavlov did not distinguish between URs and instincts, but he preferred the former term. Phylogenetically the URs emerged out of well-established conditional reflexes during the development of higher organisms. An outgrowth of the experimental conditioning procedure, developed during the second decade of this century, was the observation and delineation of new URs. While studying human nervous and psychiatric disorders in the 1930s, Pavlov elucidated other URs. Pavlov identified 13 major URs, but he failed to formulate an exhaustive classification scheme of URs.