Over the past few years a common concern of many branches of psychology has been the nature of cognitive representation, or the structure of mind. On a simple level, theories about this can be characterized according to the kind of analogies that they propose as models of thought. Thus, to G. Kelly (1955), man is a ‘scientist’, to H. Kelley (1973) man is a ‘statistician’, to Fishbein & Ajzen (1975), man is a more or less rational ‘decision-maker’ guided by probabilistically held beliefs and evaluations, and to Piaget (1972), man is a ‘logician’. But, as Fischoff (1976) ar g ues, if we wish to devise models for man as thinker, there are other disciplines in which we can search. His own suggestion is history and there may indeed be some merit in the ‘man as historian’ model. The approach taken here, however, is to look at the philosophy of science, since in this way we may be able to extend and elaborate the rational-scientific approach that is shared by all the metaphors discussed above. The two philosophers who have most to offer in this context are Popper and Lakatos. Popper’s recent work (1972) is mainly concerned with the nature of knowledge and thus he shares his prime concern with Piaget. Just like Piaget, he has severely criticized empiricism and what he has termed ‘the bucket theory of the mind’. This evocative metaphor refers to the notion that ‘facts’ can somehow directly impinge on a person’s mind, that they accumulate in his bucket, and that the function of theories is to explain the contents. In this view, facts are ‘givens’, which theories must attempt to explain. By way of contrast, Popper has emphasized that facts, observations, sensations and perceptions are not givens but constructions. Facts are simply very well established theories which may ultimately be inadequate; observation is necessarily selective and a theory is needed as a guide. Just as observations presuppose theories, so do our sense systems embody theories (in this case about the wavebands of energy that are relevant to our survival). This means that what we choose to describe as elements in a particular construction depends upon the level and kind of theory that we are dealing with: the elements in perceptual theories will be sensations, the elements in conceptual theories will be beliefs, and so on. This idea, that constructions exist on many levels, suggests a conception of the child’s mind as a hierarchy of interlocking theories: micro-theories (implicit in our sense organs), meso-theories (those concerned with what aspects of our sense experience we attend to), and macro-theories (those concerned with the way the world works). Theories at all levels share the property of construction and are all interlinked.