For capacity limits, the efficient causes could be factors that influence whether an item can be retained in a working memory task. The concurrent memory load and the type of load would be such factors. A model of working memory would be a description of what its parts are and how each one operates. Material causes are substrates or mechanisms. In the case of working memory capacity, what are most relevant are the brain mechanisms that permit it. Final causes are functions; they are teleological. For final causes, one could discuss reasons why it was important in evolution for the capacity to be as large as it is, and perhaps benefits of the capacity not being even larger. There are similar limits in the number of chunks that can be retained in either vision or audition, for either verbal or nonverbal materials. However, one must be careful in drawing inferences from those findings regarding the nature of the capacity limits.