Similarity and categorization are usually formulated as operations over a set of available features at time t of conceptual development. In this paper, we discuss possible constraints on the development of the feature space such as the history of categorization, perceptual biases in the selection and the construction of features, and the role of higherlevel knowledge and beliefs. The history of categorization is defined as the feature vocabulary and the set of concepts people have acquired as a result of representing and categorizing objects. Perceptual and developmental biases refer to the salience of specific aspects of the stimuli which influence the selection and the construction of features at different stages of development. The constraining role of general knowledge on the selection and the creation of features is also discussed. It is argued that theories may need the constraints they are supposed to provide. We conclude by discussing issues related to the interactions of perce...

x- treme from Fodor; she argues that albiexical concepts can be derived by combinations of twenty -three universal, innate primitives. A variety of evidence supports the nativist position. Take the concept of object as a case in point. By objects, I mean bounded, coherent wholes that endure through time and move on spatio-temporally continuous paths. Two extremely convincing lines of argument show this concept to 89 be largely innate. The first is direct empirical evidence demonstrating it in infants as young as two to four months. The second derives from learnability considerations. If one wants to argue that two-month-olds have constructed the concept of an object, one must show, in principle, how they could have done so. From what primitives, and on what evi- dence? Not for lack of trying, nobody has ever shown how this concept could be formed out of some prior set of primitives. What would lead an organism existing in a Quinean perceptual quality space, sensitive only t

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We report on a study with 65 middle-school students who learned about the concept of diffusion through observation. We manipulated two factors: the number of observers, solo vs. dyad, and the type of video students observed, tutorial dialogue vs. monologue. Our findings show that dyad observers learn significantly better than solo observers, and that for certain types of questions, observing dialogue results in better learning gains, as compared to observing monologue.