In analysing the Zinkernagel and Hengartner's ‘Credo 2004,’ Anderson introduces his ‘development-context model’ for the immunity-tolerance discrimination. He compares this model with the ‘geographical model of Credo 2004’ and our ‘time-based two-signal model’. The discussion here deals with the advantages and limitations of the Anderson model considered largely at the level of principle. A meaningful discussion requires that we agree on the principle which separates the pathway of the effector output into two decision steps, the sorting of the repertoire and the regulation of effector class. The mechanism for the sorting of the repertoire is what might be referred to as the Self–Nonself discrimination. The black box approach, antigen-in, effector response-out, is what is referred to as the immunity-tolerance discrimination which includes the sorting of the repertoire. If this point of principle is accepted then we are left with a ‘time-based two signal default model’.