The “A-not-B” error is consistent with an old memory principle, Jost's Law. Quantitative properties of the effect can be explained by a dynamic model for habituation that is also consistent with Jost. Piaget was well aware of the resemblance between adult memory errors and the “A-not-B” effect and, contrary to their assertions, Thelen et al.'s analysis of the object concept is much the same as his, though couched in different language.