some commentaries suggest that the attention-deficit/hyperactivity disorder (adhd) theory of this condition does not explain enough. because the theory includes parameters of the delay gradient that vary across individuals and developmental modulation of behavioral outcomes by different environments, it accommodates a wide range of manifestations of adhd symptoms. thus, the argument could instead be made that the theory allows too many degrees of freedom. for many purposes, behavior is better defined in terms of function (e.g., consequences) than in terms of structure (e.g., muscle movements), so cognition is treated here as a variety of behavior rather than as a different category of phenomena. the commentaries are discussed in the context of these and other distinctions, including those between association and selection, between operant and respondent behavior, and between fundamental processes and those that are derivative. other issues include: prosthetic environments, rapidity of developmental change, the concept of inhibition, the form of the delay gradient, and possible directions for experimental research.