Rosenthal is best known for his higher-order-thought theory of consciousness. He argues that no mental state is conscious if one is not aware of that state; so a mental state is conscious only if one is aware of oneself as being in that state. And he argues that we are aware of mental states that are conscious by having a thought that one is in that state. These higher-order thoughts are seldom conscious thoughts, and they are distinct from the states they are about. The standard mark in everyday contexts and in experimental psychology that distinguishes conscious mental states is that an individual can report the state; we are unable to report mental states that aren't conscious. Rosenthal argues that it's the higher-order thoughts that accompany conscious states that enable us to make such reports; because we have no such higher-order awareness of mental states that aren't conscious, we can't report them. Rosenthal has also argued that the higher-order-thought theory fits well with recent findings in psychology and neuropsychology.
Quality-space theory
Since mental states are conscious only if one is aware of those states, every mental state can occur without being conscious. That includes states that exhibit mental qualities, such as the perceptions and bodily sensations that occur in subliminal perception and in blindsight. This view goes against many contemporary views, on which mental qualities are tied inextricably to consciousness. So Rosenthal has developed a "quality-space theory" of the mental qualities, which explains what qualitative mental states without appeal to consciousness. All mental qualities are individuated by their positions in a quality space that pertains to their sensory modality. More specifically, we can define a quality space of the physical perceptible properties that mental qualities enable access to, relying just on the ability to discriminate among those perceptible properties. Mental qualities, then, are the properties of mental states that make that discriminative capacity possible; so they are fixed by the positions they occupy in a quality space corresponding to the quality space of accessed perceptible properties. Because the quality spaces of perceptible properties are determined solely by the ability to discriminate among sample properties, independently of whether the perceptual states are conscious, mental qualities themselves can occur without being conscious. They occur consciously only when one is aware of oneself as being in the relevant qualitative state, on the HOT theory, when one has the relevant HOT.
Relation to other theories of consciousness
Rosenthal's HOT theory of consciousness resembles in some ways the traditional inner-sense theory, on which we are aware of conscious states by perceiving them. This theory has also come to be known as higher-order perception. Rosenthal's theory also resembles somewhat the theory of Franz Brentano, on which our awareness of our own mental states is intrinsic to those states. But the HOT theory avoids various difficulties that face the inner-sense theory, e.g., explaining what sensory modality the higher-order sensing could have. And because the HOTs Rosenthal posits, unlike Brentano's intrinsic awareness, are external to the mental states they make one conscious of, the theory avoids difficulties Brentano's theory encounters about the individuation of mental states, as well as neuropsychological evidence that mental states occur measurably before they are available for awareness. Global-workspace theories posit that mental states are conscious in virtue of the wide availability of their content to various processing areas in the brain. The HOT theory has an advantage over such theories that it can explain the occurrence of conscious state whose content is not thus available, such as relatively peripheral perceptions and thoughts, and unconscious states whose content is widely available, such as repressed psychological states. First-order theories, such as Dretske's and Ned Block's, deny that a mental state's being conscious consists in some awareness of that state, and so have no easy way to explain the difference between conscious states and mental states that are not conscious. Rosenthal's HOT theory provides an intuitively natural way to do so.
Current research
Rosenthal is currently working on function of mental states' being conscious, which he argues is minimal, and on explaining why mental states do ever occur consciously if little utility results from their being conscious. He argues that the factors that explain why qualitative states, such as perceptions, often occur consciously are different from the considerations that explain why thoughts and other intentional states often occur consciously. He gave this talk, "Consciousness and its Function" at the Online Consciousness Conference. Rosenthal has also written extensively about the connection between consciousness, thought, and speech, and has edited several anthologies.
