A comparative perspective on reasoning about possibility

Abstract

As humans we spend considerable effort contemplating possibilities, whether planning for the future or pondering on the past, reasoning about what is possible is an essential aspect of our lives. For that reason, it is relevant to ask whether we are unique in our ability to consider multiple possible futures or pasts. Across four experimental chapters, I attempt to test some of the explicit claims laid out by the varying models of non-linguistic reasoning. In Chapter 2, I aimed to test Leahy and Carey’s (2020) minimal model of possibility. I developed a novel paradigm, post-decision wagering, and used it to demonstrate that great apes reason about the existence of multiple, incompatible possibilities. In Chapter 3, I modified the post-decision wagering paradigm, to test whether great apes were able to reason via the disjunctive syllogism. Finding that, if given information about the unchosen cup, subjects were able to adaptively choose between their original choice and a fractional reward. In Chapter 4, I tested all four great ape species using Mody and Carey’s (2016) classic 4-cup disjunctive syllogism task and Ferrigno et al.’s (2021) modification. Apes switched adaptively in-line with logical reasoning, but performance was poor compared to the 2-cup variant and, when I included additional control trials, subjects failed to choose above chance levels. Chapter 5 explored whether chimpanzees were curious about counterfactuals using a modification of Call and Carpenter’s (2001) 3-tube paradigm. Showing that, after being given a choice between 2 of 3 differentially baited tubes, subjects were more likely to check the contents of the unchosen than the unavailable tube. Finally, Chapter 6 discusses how these findings contribute to our understanding of how great apes reason about possibility. I explore whether our data support any of the previously proposed hypotheses and why performance breaks down in the 4-cup tasks.