His principal interests include philosophy of mind, philosophy of language, philosophy of linguistics, epistemology, and metaphysics. The author of eight books and over 150 articles Lycan is an advocate of the version of functionalism, known as homuncular functionalism. Lycan is an unapologetic realist and physicalist about almost everything – mental states, epistemic justification, linguistic meaning, perception, moral properties, aesthetic properties, and more. This philosophical tapestry is held together by several common threads, which among others include: the view that persons are complex systemscomposed of sub-personal systems, sub-sub-personal systems, etc., all the way down to sub-atomic functions; the view that natural teleology is ubiquitous, that there exists a non-ideal but nonetheless discernible evolved fit between human capacities and the environment; and the view that meaning in natural language manifests a logical form that at its core is truth-functional. These common threads are prominent in Lycan's theories of mind. Mental states are type-identical to functional states instantiated in sub- or sub-sub-personal systems, where the function of any such state is determined by its natural teleology. This is Lycan's Homuncular Functionalism. The appeal to teleology dissolves problems with earlier functionalist theories, especially those concerning consciousness. A correct theory of consciousness, on Lycan's view, comprises multiple parts corresponding to the multiple phenomena to which the term ‘consciousness’ is applied. Thus, Lycan defends a higher-order, inner sense theory of awareness, according to which attention mechanisms have the function of monitoring and integrating lower level psychological processes; a pronominal theory of subjectivity, according to which the subjective or perspectival nature of conscious states is a product of the utterly unique semantic role of introspective mental concepts ; a representational theory of qualia, according to which sensory qualities are the intentional objects of sensory representations; and more. These same threads are prominent in Lycan's account of knowledge, including his account of belief acquisition and epistemic justification. To acquire a new belief is to acquire a new real, internal representation instantiated in one's functional architecture. And a new belief is justified if it, better than its competitors, increases the explanatory coherence of the person's entire belief set. This is Lycan's explanationist theory of justification. Canons of epistemic value used to identify degrees of explanatory coherence are themselves justified by appeal to natural teleology. In defense of his view, Lycan critically assesses major competitors and other views. Meaning in natural language, including the meaning of indicative conditionals, is explained by Lycan in truth-theoretic terms. Here, too, the requisite psycho-linguistic machinery coheres with Lycan's Homuncular Functionalism and, more generally, with the above common threads. Along with Robert Adams, Lycan considers David Kellogg Lewis's notion of possible worlds to be metaphysically extravagant and suggests in its place an actualist interpretation of possible worlds as consistent, maximally complete sets of descriptions or propositions about the world, so that a "possible world" is conceived of as a complete description of a way the world could be – rather than a world which is that way.
