Let's start simple. What controls, say, walking in humans? The standard view, which I expect you to agree with at this point, is that the explanation of both the universality of walking by humans and the variation in forms of it is relatively unprob­ lematical. It involves two distinct forms of cause. There is an underlying propensity for humans to walk due to the opera­ tion of an in-built genetic programme applying to all members of the species, being independent of learning, not based upon specific environmental events, and not dependent upon inter­ actions between internal and external factors. It is 'innate', part of 'nature'. There are, on the other hand, a variety of forms of walking to be observed, the variations being due to experien­ tial factors, shoes worn, fitness, load carried, cultural norms and so on, the 'nurture' side of the coin. Thus any given instance of walking can be accounted for by a consideration of the relative contribution of the two sorts of causal factors and their interaction. The same general argument applies to every human ability. It is a model for all development. It is the domi­ nant paradigm.