Summary: In this paper we consider data from a study in which students shift from linear to quadratic equations in ways that do not conform to established theoretical frameworks. In solving linear equations, the students did not exhibit the ‘didactic cut’ of {\it E. Filloy} and {\it T. Rojano} [Learn. Math. 9, No. 2, 19‒25 (1989; ME 1990b.00884)] or the subtleties arising from conceiving an equation as a balance. Instead they used ‘procedural embodiments’, shifting terms around with added ‘rules’ to obtain the correct answer. Faced with quadratic equations, the students learn to apply the formula with little success. The interpretation of this data requires earlier theories to be seen within a more comprehensive framework that places them in an evolving context. We use the developing framework of three worlds of mathematics, based fundamentally on human perceptions and actions and their consequences, at each stage taking into account the experiences that students have ‘met-before’. These experiences may be supportive in new contexts, encouraging pleasurable generalization, or problematic, causing confusion and even mathematical anxiety. We consider how this framework explains and predicts the observed data, how it evolves from earlier theories, and how it gives insights that have both theoretical and practical consequences.