While Gottlieb recognized the significance of biological factors for behavioral development, the system of psychology he developed did not cast the discipline as a purely biological science. Rather, genes, brains, hormones were understood by him as participating, rather than causal factors in behavioral origins. He worked from two basic principles that he said he became aware of in graduate school: the importance of prenatal development and the bidirectional nature of structure-function relations. Development was, for him, epigenetic, although it was probabilistic and not predetermined. He was, therefore, against biological determinism and strict genetic reductionism. Behavioral origins have an experiential basis, although he understood much of that experience to be non-obvious and arduous to discern.