Children know the fun of throwing a ball into the sea, only to watch the waves fling it back. Jennifer Mather and Roland Anderson at the Seattle Aquarium were surprised to find octopuses playing similar games. Their toy was a floating pill bottle, which they were free to ignore or explore as they wished. Six of the aquarium’s octopuses soon lost interest, but two showed childlike curiosity, pushing it with their arms or shooting jets of water to move it against the tank’s current. It is hard to interpret this as anything other than play, which many researchers argue requires some form of conscious awareness.

Many animals exhibit behaviours similarly suggestive of an inner life. Conscious creatures may include our primate cousins, cetaceans and corvids – and potentially many invertebrates, including bees, spiders and cephalopods such as octopuses, cuttlefish and squid. The challenge, of course, is to understand how the inner lives of these creatures differ from our own.

In the past, scientists spoke about “levels of consciousness”, as if there were a hierarchy with humans on top. But in a paper published in 2020, Jonathan Birch, a philosopher at the London School of Economics and his colleagues argue that we would do better to consider five separate elements of conscious experience. The first …