An elaborate theory of learning (1) formulated in 1940 by the US psychologist Clark L. Hull (1884–1952), published in his book Principles of Behavior (1943), and presented in a modified form in A Behavior System (1952). In the final version, the independent variables are stimulus energy (S), work involved in response (W), number of reinforcements (N), and difference between the current stimulus and the stimulus used in training (d); the dependent variables are reaction latency (StR), response amplitude (A), and number of unreinforced responses required to produce extinction (n); and the theory also has scores of intervening variables. The gist of the theory is as follows: learning is represented by habit strength (SHR), the S and the R nestling on either side of the H to symbolize the role of a habit in linking stimuli to responses, and SHR is a logarithmic function of n. Generalized habit strength (SH̄R) is determined jointly by SHR and d. Generalized reaction potential (SE̲R) is a multiplicative function of SH̄R and three other intervening variables: SE̲R = SH̄R × D × K × V, where D is drive strength, K is incentive motivation (determined in turn by W) and V is stimulus intensity dynamism (determined in turn by S). Net reaction potential is defined as follows: SE̲R = SE̲R − SİR, where SİR is aggregate inhibitory potential (determined in turn by d and W acting via various intervening variables). Finally, SE̲R interacts with behavioural oscillation (SOR) and reaction threshold (SLR) to determine the dependent variables StR, A, and n.