 Attention is involved in the selective directedness of our mental lives. The nature of this selectivity is one of the principal points of disagreement between the extant theories of attention. Some of the most influential theories treat the selectivity of attention as resulting from limitations in the brain’s capacity to process the complex properties of multiple perceivable stimuli. Other theories take the selectivity of attention to be the result of limitations in the thinking subject’s capacity to consciously entertain multiple trains of thought. A third group attempt to account for attention’s selectivity in ways that need not make any reference to limitations in capacity. These latter theories relate the selectivity of attention to the selectivity required to maintain a single coherent course of action, to the weighting of sensory information in accordance with its expected precision, or to competition between mutually inhibitory streams of processing.