What is attention? That is the question that those who investigate the nature of attention try to answer. Theories about the nature of attention can be classified along several dimensions. First, their metaphysical level. Here we can distinguish reductivist theories that identify what attention is at the level of neuronal, or sub-personal psychological processes. Anti-reductivists hold that the nature attention is fundamentally found at level of the whole subject. Second, there is how those theories treat the unity of attention as a phenomenon. Unity views hold that attention has a unified nature, while disunity views hold that, in some way, attention is not a unified thing. Third, there is which metaphysical category attention is placed in. Views on attention disagree whether it falls fundamentally in the category of a type of mechanism, an adverbial modification, an activity, a resource, o.a. Some theories of the nature of attention relate it to action, consciousness, motor behavior, Bayesian optimisation, or embodiment. A discussion that is closely related to the nature of attention concerns the biological or psychological function of attention.