Developmentalists have overlooked the fundamental problem of the real impermanence of things. The question of the development of the rational understanding of impermanence is as yet unposed, despite the fact that the metaphor of impermanence is central to both Piagetian and neo-nativist accounts of representation. By claiming that young infants lacking representation treat absent objects as if they were destroyed or ‘impermanent’, Piaget created the impression that the development of representation resolves the impermanence problem. Discussions of conservation compound this confusion by interpreting young children’s beliefs in the destructibility of objects as failures of conservation, as if the development of the conservation concept could somehow do away with the real vulnerability of things. These confusions prevent consideration of the fundamental role that impermanence concepts play in many aspects of human experience and subjectivity. It is proposed here that the development of the rational concept of impermanence must be viewed as distinct from the development of conservation and logico-mathematical reversibility as conceived by Piaget.