6 ideas
Humphrey studied Helen, a monkey whose visual cortex had been removed. She could still navigate and grab objects accurately while apparently lacking visual experience. He takes this, along with human blindsight patients, as evidence that perception (information about the world) and sensation (felt experience of stimulation) are separable processes, and that only sensation carries phenomenal quality.
Sensation began as an overt evaluative response, a 'wriggle' of the body toward or away from stimulation at the skin. Over evolution these responses became internalized into efference-copy loops that target only the sensory map and no longer the body surface. Feeling is therefore something the subject actively does ('sentition'), not something it passively receives.
Humphrey argues that the privatized sensory loop stabilized into a self-sustaining attractor, which he calls an 'ipsundrum.' It generates the appearance of ineffable, extra-physical qualities, a magical-seeming illusion the brain stages for itself. Consciousness on this view is a representation designed to seem mysterious, which explains why the hard problem feels hard.
Phenomenal experience has no direct computational payoff, since zombie-like perception could guide behavior. Its adaptive value is motivational and existential. A creature that experiences itself as a rich, special, enduring self takes a stronger interest in staying alive, invests in its own future, and values selfhood as an end in itself.
To test whether an animal is sentient, do not ask whether it processes information or reacts to harm. Ask whether it shows behavior that makes sense only if it values sensation for its own sake. Examples are sensation-seeking, play, curiosity about its own experience, and treating itself as a self with interests.
Humphrey concludes that phenomenal consciousness is probably confined to mammals and birds, whose warm-bloodedness supports the fast, stable neural loops sentience requires. Octopuses, fish, insects, and current AI are likely very clever without anything it is like to be them. Sentience is therefore a late, rare evolutionary invention, not a continuous gradient running throughout life.