The problem is compounded by the fact that these organismic processes are not directly observable and hence must be inferred. Early behaviourists, influenced by the empiricist view that knowledge is dependent on sensory experience and the positivist requirement that science be based on observation, were concerned that any concepts postulated should be tied to observations. According to the logical positivists' verification principle, the meaningfulness of a proposition depends on being able to specify conditions for determining its truth value. Hence operational definitions, which specified such conditions for theoretical terms, were adopted in physics (Bridgman, 1927) and extended to psychology by Stevens (1939). On the strict version theoretical terms are completely reducible, and identical in meaning, to observations; according to a weaker version they must be related to observations but these do not necessarily exhaust their meaning. For example, hunger drive might be defined purely in terms of hours of food deprivation, or might be taken to refer to some internal processes not directly observed.