Rosenthal’s Higher-Order Thought

Philosopher David Rosenthal proposes “a higher-order-thought [HOT] theory of consciousness, according to which a sensation, thought, or other mental state is conscious if one has a higher-order thought that one is in that state” (Rosenthal, 2005). In other words, this “implies that a state’s being conscious requires that one be in some way aware of being in that state. And that’s the core of any higher-order (HO) theory of consciousness” (Rosenthal, 2012).

Core Commitments of HOT

While higher-order theorists all hold that “a state is conscious only if one is in some way aware of the state,” they differ “about what that way is.” According to Rosenthal’s HOT theory, “one is aware of one’s conscious state by having a thought that one is in that state. [But] that HOT need not itself be conscious. And to capture the subjective immediacy of the way we’re aware of conscious states, the HOT must not seem subjectively to rely on an inference or observation.”

Rosenthal argues that a theory of consciousness must explain “how mental states that are conscious differ from those that are not conscious. Otherwise, we won’t have said why it is that the conscious ones are conscious.” He critiques first-order (FO) theories of consciousness, “which see a state’s being conscious as independent of any substantive higher-order awareness (HOA) of the state,” whether Fred Dretske’s psychological account or Ned Block’s neural mark. Rosenthal stresses that FO theories “must explain how mental states that are conscious differ from those that are not. Mere co-occurrence of a preferred feature with the states we count as conscious is not enough—unless it also explains why it’s that feature that makes the difference between a state’s being conscious and not (Rosenthal, 2012).

In support of his HOT theory, Rosenthal stresses that there is no need for introspective access; there is no need to introspectively reflect on a mental state for it to be conscious. The HOT can occur unconsciously and still render first-order states conscious (Rosenthal, 1997).

Homomorphism Theory and Qualitative Consciousness

Rosenthal also proposes, as a background assumption, a related “homomorphism” theory of mental qualities and qualitative consciousness, which shows how mental representations relate to external properties or states of affairs, thus grounding representation in structural relations. This is achieved by examining structural similarities or mappings (homomorphisms) between the structure of the represented world and the representational structure in the mind. In this way, Rosenthal’s homomorphism theory undergirds his HOT theory of consciousness, because the content of the first-order state (the represented content) is shaped structurally by the homomorphic mapping to the world. (The HOT need not mirror the structure precisely, but the first-order representation is affected by that mapping.)

As Rosenthal explains, “A crucial feature of homomorphism theory is that it individuates and taxonomizes mental qualities independently of the way we're conscious of them, and indeed independently of our being conscious of them at all. So the theory accommodates the qualitative character not only of conscious sensations and perceptions, but also of those which fall outside our stream of consciousness.” Crucially, it is Rosenthal’s naturalistic claim that, “because this account of mental qualities makes no appeal to consciousness, it enables us to dispel such traditional quandaries as the alleged conceivability of undetectable quality inversion, and to disarm various apparent obstacles to explaining qualitative consciousness and understanding its nature” (Rosenthal, 2005).

Critiques and Responses

Responding to the critique that in HOT theory, the HOA that occurs with conscious states is not substantive—maybe, the anti-HOT argument goes, being conscious of a conscious state is like smiling a smile—Rosenthal argues that “smiling a smile is simply smiling—so it can't provide a contrast like that between a state’s being conscious or not. No such ‘deflationary’ account of the HOA can explain how conscious states differ from mental states that aren’t conscious. For that, we need a substantive awareness of the state” (Rosenthal, 2012).

With respect to neural correlates of consciousness (NCC), “HO theories do not conflict with finding neural markers of mental states’ being conscious. Indeed, they may well help guide the search for an NCC. That’s because of the constraint that HO theories place on any theory and the NCC. HO theories reflect the observation that no state is conscious unless one is in some way aware of it. So, a satisfactory NCC should point to or provide a mechanism that subserves or gives rise to that HOA” (Rosenthal, 2012).

Responding to the critique that HOT theory adds an unneeded “extra ingredient,” Rosenthal and co-authors Richard Brown and Joseph LeDoux write, “This HO awareness is central to our commonsense understanding of what it is for a psychological state to be conscious… So the HO awareness that HOT theory posits as accompanying every conscious state is by no means an extra ingredient. And HO theorists have no difficulty describing why that posit is crucial. The HO awareness is needed to explain how conscious states differ from psychological states that aren’t conscious.” They conclude, “Given the failure of the list of features to differentiate conscious from unconscious psychological states, the need for an appeal to the HO awareness posited by HO theories is all the more pressing. It is by no means an ‘extra ingredient’” (Brown et al., 2021).

Quality-Space Theory (QST)

Even granting HOT, there is still the ever-looming problem of qualia, our inner awareness of felt experience. To explain qualia, Rosenberg offers his Quality-Space Theory (QST) to account for the subjective feel or qualitative character of experiences. This means that he views consciousness per se and qualia as distinct but related aspects of conscious experience.

QST’s core idea is that mental qualities (qualia) can be understood functionally in terms of their roles in perceptually discriminating stimulus properties. These qualities are organized within abstract “quality spaces” that structurally mirror the relationships between the corresponding perceptible properties in the external world and which are determined by how well an individual can discriminate among stimuli. According to QST, mental qualities represent their corresponding perceptible properties in a manner that is nonconceptual and holistic (Rosenthal, 2015).

Rosenthal claims that QST offers a realist, wholly naturalistic account of qualia, grounding them in our perceptual abilities and the structure of the world, while maintaining their distinctness from consciousness. This allows for qualitative experiences that are not necessarily consciously accessible, and provides a framework for how qualitative experiences can exist independently of conscious awareness, which aligns with phenomena like subliminal perception or unattended sensory input.

These mental qualities do not require direct awareness to exist. In other words, a mental quality (i.e., a sight, a sound) can occur without being conscious—which aligns with Rosenthal's HOT theory that consciousness arises when a higher-order thought targets a mental state (Rosenthal, 2015).