AbstractMacnamara and Austin argue that Piaget seriously misinterpreted the significance of children's documented difficulties in correctly responding to tasks involving conservation of quantity. In this paper, I have tried to show that they have made some basic errors in their analysis of the Piagetian position and that their alternative theory of how children learn to conserve is both inconsistent with empirical evidence and epistemologically untenable.In Physics and Plasticine, Macnamara and Austin present a critique of the Piagetian theory of development that focusses on the specific notion of conservation of matter. They claim that while the empirical data is correct, the Piagetian interpretation of these results is simply incorrect and that it makes more sense to suppose that results showing that young children cannot respond correctly to problems of conservation of matter can be better explained by their lack of a correct physical theory, which could and should be supplied to them earlier.Macnamara and Austin present two separate arguments. The first is a set of critiques of Piaget's analysis, while the second represents an attempt to outline how children might come to understand conservation. Let us start by examining the latter. Macnamara and Austin make two basic claims about children. The first major claim (which is implicit to their argument) is that children and adults reason in a fundamentally similar way. The second claim is that the only thing that children lack in order to understand the conservation of solid quantity is an appropriate physical theory. The first claim, i.e. that in some fundamental sense adults reason in the same way as children, amounts to saying that some basic and interesting components of adult reasoning, abilities are essentially innate, since if they were learned or developed, it would then be the case that some children reasoned differently than adults. One can understand why educators, who are faced with the reality of children's reasoning, might find such a claim startling, but for the sake of argument, let us suppose that some basic capacities of adult reasoning are in fact shared by young children. In line with Macnamara and Austin's analysis, let us suppose that this means that young children have a capacity to make inferences that is the same as that of adults. Given this useful ability, how would children learn the notion of conservation of matter. Macnamara and Austin suggest two related ways. The first way involves learning sufficient physical theory in order to be able to deduce that matter is conserved under changes of shape. This however is a hazardous procedure, as a brief examination of the physicist's approach to conservation indicates. The authors' analysis of conservation of volume (which is a much later development than conservation of quantity and should be clearly differentiated from this) is particularly interesting. We first find out here that in the real world, volume is not in fact conserved as a general rule. Then we are told that the physical theory that must be learned to understand how volume works involves Poisson ratios, impermeability, and certainly a host of other concepts whose complexity would drive all but an undergraduate physics major to despair. Surely, Macnamara and Austin are not suggesting that children must learn this kind of theory, since if true it would be surprising to find anyone below the age of 20 (and only a selected subset of these) who understood conservation.Another and seemingly simpler suggestion in this line concerns the developmentally earlier notion of conservation of quantity. Here we are told that our physicist would demonstrate the fact that the quantity of matter (its mass) remains the same by weighing a piece of plasticine before and after its conversion into a sausage. Our physicist can use such a seemingly simple procedure since she or he knows that weight is directly related to mass. But how does our physicist know this? …