1. in scientific theory, the procedure of explaining a complex phenomenon by reducing it to simple, elemental units.

2. the belief that such a procedure is appropriate to a science dealing with psychological phenomena, which are explained by reduction to simple elements, such as basic sensations or elementary reflexes. Both psychological structuralism and behaviorism have been described as elementarist approaches. Also called elementalism; elementism. See also atomism; molecular. —elementarist adj.