At this point one might object: How can we know that animal dreams are meaningful to them, and not merely jumbles of random sensation? Animals cannot keep journals or recount their dreams to researchers. (Most cannot, anyhow—a gorilla named Michael whose mother was butchered by traffickers in the forests of Cameroon would have recurring nightmares. He described them in ASL, signing, “Bad people kill gorillas.”) Skeptics of animal cognition often describe animal actions behavioristically, as learned reactions to a stimulus. Could the same be going on in sleep? Peña-Guzmán rejects this interpretation by pointing out that a dreaming animal is not reacting to anything. A sleeping rat navigates a dream-conjured maze while its body lies still in its cage. The animal is responding only to the world that a mind—its own mind—has invented.

For the nonphilosophers among us, all this debate over consciousness might seem a bit silly. I look at my dog, I call her name, and she looks back at me. Sometimes she comes when I call, bounding over in expectation of a treat or affection; sometimes she chooses to ignore me and go about her doggy ways. That she has a mind of her own seems clear enough.

And yet most of us, pet owners included, tend not to treat animals as fellow minds. (How often have I heard the tired claim from trainers that dogs are driven purely by pack instinct.) For billions of humans on this Earth, animals are food, labor, or material. To recognize that animals have consciousness would be a first step toward considering them in possession of inviolable rights alongside our own. A court in New York earlier this year declined to grant personhood to Happy, an elephant at the Bronx Zoo, despite one judge’s admission that Happy is “an intelligent, autonomous being who should be treated with respect and dignity.” But we needn’t equate animals with humans to recognize that their subjective experience can be as vibrant as our own. Understanding that they might feel, think, and dream makes it harder to imagine 800 million years of animal life as wholly devoid of mind—a house empty and dark, with the lights switched off.