Kenneth Spence (1936, 1937) formalized a quantitative, elemental approach to association theory that has had a broad and dominating influence on learning theory for many years. A set of challenges to the basic approach has spurred the subsequent evolution of elemental theory in various ways. Four of the challenges and some resulting theoretical accommodations are described in the context of Pavlovian conditioning. The evolution involves departures from important specifics of Spence's theory, but is viewed as demonstrating the utility of the basic, elemental approach that is one of his legacies.