From an organismic, probabilistic epigenetic viewpoint, development arises from a dynamic interaction between qualitatively different, yet totally interdependent nature and nurture variables. The syntheses defining this interaction involve the quality and timing of each source providing a basis of these parameters in the other. Covariation among the parameters shows interorganism variability in ecologically valid milieus. Organisms consequently develop lawful characteristics of individuality; this individuality is a basis of further differential interactions, both at a within-organism level and between the individual and its extraorganism experience. These latter interactions are also dynamic. In reaction to organism individuality differential reactions are promoted in other organisms; these feed back to provide a further differential basis of development. The presence of such circular functions means that organism-organism (social) relations exist in states of mutual alteration, in dialectical interaction. Suitable empirical procedures exist for describing historical/cultural, social, individual, and temporal contributions to development, and should be applied in order to detail how organisms in dynamic interaction are at one and the same time both products of their social world and producers of it.