Chrysippus’s dog is a great example of apparent inferential reasoning. Fortunately, we now have tests of inferential reasoning. In the 1990s, the psychologists David and Ann Premack presented their chimpanzee, Sarah, with two boxes, putting an apple in one and a banana in the other. After a few minutes, Sarah would watch one of the experimenters munch on either an apple or a banana. This experimenter then left the room, and Sarah was given a chance to inspect the boxes.

She faced an interesting dilemma, since she had not seen how the experimenter had obtained his fruit. Invariably, however, she would go to the box with the fruit that the experimenter had not eaten. She must have concluded that the experimenter had taken his fruit from the corresponding box and that the second box would still contain its original fruit. Most animals don’t make any such assumptions, the Premacks note; they just see an experimenter consume fruit. Chimpanzees, by contrast, always try to figure out the order of events, looking for logic, filling in the blanks.

Read: A journey into the animal mind

In another test, apes were presented with two covered cups after they had learned that only one would be baited with grapes. Both cups were covered and shaken. As expected, the apes preferred the cup in which they could hear the grapes make noise. But then the experimenter shook just the empty cup, which obviously made no sound. The apes picked the other cup.

I once watched another such causal inference unfold at the Burgers’ Zoo when the chimpanzees in the indoor colony watched us carry a crate full of grapefruit, which they find delicious, through a door that went outside onto their island. They seemed interested enough. But when we returned to the building with an empty crate, pandemonium broke out. As soon as they saw that the fruit was gone, 25 apes burst out hooting and hollering in a most festive mood.

Animal consciousness is hard to investigate, but we are getting close by exploring examples of reasoning, such as those given above, that we humans cannot perform unconsciously. We cannot plan a party without consciously thinking about all the things we need; the same must apply when animals plan for the future. The latest neuroscience suggests that consciousness is an adaptive capacity that allows us both to imagine the future and to connect the dots between past events. We are said to have a “workspace” in the brain where we consciously store one event until another one comes along.

Take, for example, taste aversion in rats. It is well known that rats avoid certain toxic foods, even if they don’t become nauseous until hours later. Simple association fails to explain this. Could it be that rats consciously go over the recent past in their minds, thinking back to every food encounter to determine which one was most likely to have made them sick? We certainly do so ourselves after food poisoning and gag at the mere thought of the particular food or restaurant that we believe caused a shock to our digestive system.