 The instances of attention differ along several dimensions of variation. In some of its instances attention is a perceptual phenomenon. In other instances it is a phenomenon related to action. In some instances the selectivity of attention is voluntary. In other instances it is driven, quite independently of the subject’s volition, by the high salience of attention-grabbing items in the perceptual field. The difficulty of giving a unified theory of attention that applies to attention’s voluntary and involuntary instances, and to its perceptual and enactive instances, makes attention a topic of philosophical interest in its own right.