How does sentience relate to perception? Let me tell a story. Fifty years ago, when I was a Ph.D. student in Cambridge, a research project changed my life. There was a monkey in our lab, called Helen, who had undergone surgery that removed the visual cortex of her brain. When I first met Helen, a year after the operation, it appeared she had been rendered completely blind. Curiously, there are other animals, frogs for example, that have no visual cortex to start with—and they can see perfectly well. Then, why not Helen?

While my supervisor was away at a conference, I took the chance to investigate further. I sat with Helen and played with her, offering her treats for any attempt to engage with me by sight. To my delight, she began to respond. Within a few hours, I had her reaching out to take a piece of apple that I waved in front of her; within a week, she was reaching out to touch a small flashing light. This was just the beginning; five years later, she was running around a complex arena, deftly avoiding obstacles, and picking up peanuts from the floor.

A casual observer might have thought her sight had now returned to normal. Yet, I had reservations. It was hard to put my finger on what was wrong. My hunch was that Helen herself still doubted she could see. She seemed strangely unsure of herself. If she was upset or frightened, her confidence would desert her, and she would stumble about as if in the dark again.

In fact, we were on the verge of a remarkable discovery. We discovered later that a human patient, who had become blind after damage to the visual cortex, could also see in a way he himself didn’t believe. He denied he had any visual sensations, but he could still guess the position, shape and color of an object in front of his eyes. It seemed he had unconscious vision. Blindsight, as we called it later.

The discovery of blindsight was theoretically shocking. No one had ever expected there could be any such dissociation between perception and conscious sensation. After ruminating on the implications of this for understanding consciousness, I found myself asking a question out of the left field: if it’s possible to see after losing the capacity for experiencing sensations, then what is the point of having the sensations to begin with? It became the question that’s kept me busy these fifty years: What is sentience for?

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