Unobservables are hypothetical constructs, such as mental states, and physical forces including gravity and transfer of force. They are abstractions that do not take on physical form, so they cannot be perceived through any of the senses.
Unobservables are arguably the most abstract type of construct because they cannot be directly connected to physical features, although they can be correlated with observable events and objects. For example, gravity itself cannot be observed, but one can witness objects falling when released from support. One cannot directly see confidence, but it is manifested in postural cues and behaviors. Therefore, although unobservables are unavailable to the senses, they can be inferred and reasoned about. It is important to note that unobservables are undetected by senses other than the visual sense as well. That is, they cannot be heard, tasted, or felt any more than they can be seen. Unobservables are perhaps most interesting from the perspective of comparative psychologists in the sense that they can be inferred as causal forces giving rise to both physical and psychological events.
The Unobservability Hypothesis first formally posited by Vonk and Povinelli (2006) suggested that the capacity to reason about unobservables may be one of the most significant cognitive capacities to distinguish human from nonhuman minds. Vonk and Povinelli presented substantial evidence that nonhumans represent various abstractions; for example, second-order relations, numerosities, and social relationships. Yet, even abstractions that can be directly tied to sensations, such as weight, appear challenging for other apes to reason about. An extensive series of studies with a group of chimpanzees suggested that apes do not view weight as a property of objects, but, rather, respond to the kinesthetic cues that objects of different weights produce during agent-object interactions (Povinelli 2012). Abstractions that represent hypotheticals, such as death, freedom, supernatural forces etc. might plausibly be expected to pose an even greater challenge because they are not reliably correlated with consistent observable features or sensations. Vonk and Povinelli questioned whether nonhumans could reason about physical forces such as gravity, and psychological attributes, such as thoughts, beliefs, and emotions. At the time of their hypothesis, the jury was out on whether even great apes could represent others’ mental states (or their own), but the prevailing evidence suggested that they could not. In the past decade, researchers have become more amendable to the idea that other apes reason about at least some mental states in some contexts, but the ability is hardly as hegemonious as it is in humans.
When unobservables are consistently correlated with observable manifestations, such as the association between tears and crying vocalizations and sadness, it is difficult to tease apart the degree to which an organism has learned the outcomes associated with the observable features (in this case, tears and sounds of crying) or is drawing inferences based on the underlying and unobservable mental state (e.g., the emotion of sadness). Thus, the puzzle of whether nonhumans represent other minds, including thoughts, feelings, and beliefs, generally known as theory of mind (ToM), has captured the attention of psychologists for decades (Call and Tomasello 2008). Despite an immense amount of energy and resources directed to this puzzle, researchers remain divided on the question of whether other animals reason about mental states. Mental states are just one class of unobservables and arguably, the most heavily researched. Perhaps the largest gains have also been made in this area. It appears that some species, predominantly other apes and members of the corvid family, have the capacity to correctly anticipate outcomes based on others’ perspectives (Emery and Clayton 2001; Karg et al. 2015; Lurz et al. 2018), knowledge (Kaminski et al. 2008; Krupenye et al. 2016) and intentions (Buttelmann et al. 2017). However, even the most heralded studies, such as Krupenye et al.’ (2016) observation that apes correctly anticipated a human agent searching where he thought an object would be, could be explained by the apes expecting the agent to search where he had last seen the object (or otherwise attended to it). Even when chimpanzees do appear to use their own experience to project internal states upon other agents, they do not appear to do so flexibly across contexts (Karg et al. 2015) the way that even human children do by the age of 5. These experience projection tasks hold the most promise for dissociating reasoning about unobservables from reasoning about observable correlates but have produced mixed evidence even in chimpanzees (Vonk and Povinelli 2011).
The larger question of whether animals reason about unobservables more broadly remains as elusive. As with mental states, causal forces typically manifest in observable events and behaviors such as object movements, and therefore, it can be equally difficult to determine whether an organism is reasoning about the manifestation or the underlying cause. Several clever paradigms have been designed in this area as well. For example, Blaisdell et al. (2006) examined whether rats were sensitive to whether an intervening action had occurred in associating cues with outcomes, thus exhibiting Bayesian causality. Even this careful design has been critiqued, however, by authors suggesting that response competition, i.e., lever pressing, interfered with the rats’ responses in this paradigm (e.g., Dwyer et al. 2009). As with the work on ToM, the high degree of association between unobservable causal forces and their outward manifestations challenges researchers’ abilities to identify the mechanism underlying animal’s decisions.
Researchers will make little progress in determining whether other species reason about unobservables, and when this ability emerges in human ontogeny, until they disavow experimental paradigms that allow success by attending to observable correlates. Thus, the question remains central to comparative psychology as reasoning about unobservables could be the foundation for all of the distinguishing characteristics of humans, e.g., language, technology, teaching, culture, etc.
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