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Freud's psychological formulations of the development of the mental apparatus and the unpleasure-pleasure principles were
based on neo-Lamarckian evolutional theories and Fechner's constancy principle. Modern biopsychologists, especially T. C. Schneirla, provide a sounder biological substratum. According to Schneirla, species-typical behavior patterns are characterized in all animals by biphasic, functionally opposed mechanisms: approach to low-intensity stimuli, withdrawal from high-intensity stimuli. In evolution biological patterns of "approach" and "withdrawal" develop into psychological patterns of "seeking" and "avoidance." Freud's unpleasure principle was modeled on the necessity to withdraw from excessive external stimulation and to re-establish constancy. The pleasure principle presupposed a wish to re-create a situation of need gratification. The instinctual drives, an internalized motivational force, which developed in evolution as the precipitate of external stimulation, provide the energy behind the wish. The pleasure principle regulates the need to seek stimuli and to re-create by action or fantasy the experience of satisfaction through elimination of drive tension. Schneirla's evolutionary and developmental theory of biphasic processes underlying approach and withdrawal (1959) provides a more satisfactory principle than either Fechner's constancy principle (1873) or Freud's death instinct theory (1920) as the biological substratum Freud was seeking for his formulations of the regulatory principles of mental functioning.