Hasok Chang describes in Inventing Temperature how scientists dealt with the problem of measurement verification circularity when standardizing the first thermometers ever constructed. The problem can be illustrated by imagining you are the first scientist who wanted to measure the temperature of boiling water. What materials should you use to construct the measuring instrument? Once built, how do you verify your thermometer is measuring what you claim it is without circularly relying on your thermometer? Appealing to more experimentation is unhelpful because we must use a thermometer to carry out these experiments, and thermometers are what we are trying to determine the reliability of in the first place. Hasok Chang calls this the Problem of Nomic Measurement (PNM), which is defined as:
The problem of circularity in attempting to justify a measurement method that relies on an empirical law that connects the quantity to be measured with another quantity that is (more) directly observable.1 The verification of the law would require the knowledge of various values of the quantity to be measured, which one cannot reliably obtain without confidence in the method of measurement.
Stated more precisely, the PNM goes as follows:
1. We want to measure unknown quantity X.
2. Quantity X is not directly observable, so we infer it from another quantity Y, which is directly observable.
3. For this inference we need a law that expresses X as a function of Y, as follows:X = f(Y).
4. The form of this function f cannot be discovered or tested empirically because that would involve knowing the values of both Y and X, and X is the unknown variable that we are trying to measure.
My aim for this paper is to apply the PNM to an on-going debate in cross-comparative psychology about whether and to what extent non-human animals can “mentally time travel”. In 1997, Suddendorf and Corballis argued “the human ability to travel mentally in time constitutes a discontinuity between ourselves and other animals”.2 In 2002, Roberts argued non-human animals are “stuck-in-time”. Since then, a number of psychologists have defended similar claims. Endel Tulving states this hypothesis clearly:
There is no evidence that any nonhuman animals—including what we might call higher animals—ever think about what we could call subjective time…they do not seem to have the same kind of ability humans do to travel back in time in their own minds, probably because they do not need to. (Tulving, 2002, p. 2)
Call the claim that mental time travel is unique to humans Uniqueness. Naturally, Uniqueness has not gone unchallenged. One worry is that different theoretical assumptions about what counts as “mental time travel” are leading to disagreements over whether animals do or do not possess MTT. Furthermore, both sides of the debate more or less agree about the behavioral evidence, but disagree about how to interpret the evidence qua evidence for or against Uniqueness. This raises a problem of verification circularity similar to the PNM:
1. We want to measure MTT in animals
2. MTT is not directly observable, so we infer it from behavior Y, which is directly observable.
3. For this to work, we need to know how to infer MTT from behavior alone.
4. The form of this function cannot be discovered or tested empirically because that would involve knowing the unknown variable we are trying to measure (MTT).
Accordingly, my central thesis is that the question of whether animals can mentally time travel is not a purely empirical question. My argument hinges on premise (3): if psychologists have irreconcilable differences in opinion about which behaviors best express MTT, they will use the construct “mental time travel” to describe distinct phenomena and thus make different inferences from behavior to MTT. For example, if defenders of Uniqueness are using MTT as a label to describe a human autapomorphy3 but critics of Uniqueness are using MTT as a label for a core capacity shared with other animals, then they are clearly talking past each other and the debate is reduced to a semantic dispute about whether the term “MTT” is applied to “core” capacities or uniquely human traits.4 Therefore, I argue the empirical question of whether animals can in fact mentally time travel is intractable unless theorists can agree on both the connotative and denotative definitions of the term i.e. approximate agreement on the conceptual definition as well as agreement on its conditions of realization in the physical, measurable world.