The sequence is the fundamental unit of developmental analysis. Drawing on the contemporary search for the origins in infancy of later abilities, we extend the theory of sequences by introducing the concepts of prerequisite and precursor. A prerequisite is a necessary condition for a later ability. A precursor is a prerequisite ability that performs an analogous transformation on the world to the later ability. The foundational precursor to an ability marks the first explicit appearance of that ability and hence the beginning of a sequence. Attention to prerequisites and precursors is necessary for answering questions of sequence existence, membership, and order. Because these concepts are theory dependent, appropriate developmental research programs must be committed to theory building. Empirical techniques for detecting temporal orders are inadequate to determine developmental sequences.
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