Causal Inquiry in International RelationsBy Adam R. C. Humphreys and Hidemi SuganamiOxford University Press, 2025

In Causal Inquiry in International Relations, Adam Humphreys and Hidemi Suganami have made an outstanding contribution to ongoing debates about causality in our field and in the social sciences in general.[1] This is a book that will change the way a reader approaches the critical task of asking and answering causal questions, which they define as questions about whether and how an event “contributed to the production, or bringing about, of” another event (p. 14). Their central argument is that causal inquiry “hinges on the relationship between concrete causal statements which do refer to specific events and abstract statements which do not” (p. 128), where the latter statements involve propensities for one type of event to produce another under the right circumstances. Along the way, they critique major positions in the philosophy of causation and the practice of causal inference, including Bhaskarian critical realism and what they call “the culture of generalization”: “the tendency to prioritise the production of ‘general’ knowledge over knowledge of ‘particular’…facts and events” (p. 4).

Humphreys and Suganami begin their analysis with a detailed reading of David Hume’s writings on causality. Hume, in many discussions of causality, is typically presented as a defender of the “regularity view” of causation, according to which causality is nothing but an idea formed in the mind when we see a regular conjunction of events — which collapses the distinction between causation and empirical regularity, and makes the observation of empirical regularity in the world the necessary mark of a causal connection. The authors persuasively show that Hume does not actually advance this view, and that even though they themselves are not Humeans, the version of “Humean causation” that is defended by neopositivists and criticised by scientific realists is a thin caricature (p. 41). This, in turn, sets up the authors’ discussion of Roy Bhaskar’s argument that causality must be something real in the world, and not just a projection of the mind onto observable empirical regularities. The authors draw on philosopher Bas Van Fraassen’s position of “causal agnosticism” — “acknowledging that causal language is intimately bound up with how we, as human beings, relate to and make sense of the world and our place in it, but denying that it is possible to know whether it latches on to a part of the world existing independently of human thought” (p. 104) — to suggest that it is better to leave metaphysics aside[2] and focus on what we actually do when we conduct causal inquiry.

If causality is neither a gloss on empirical regularity nor the manifestation of an unobservable-but-real dispositional property of events, what is it? Humphreys and Suganami argue that we should begin by noticing that there are “two kinds of causal statement: statements which relate to specific events, located in space and time, and statements which abstract from specific events and hence possess a broader applicability” (p. 113). Where a concrete causal statement, which they further refine as an explanatory statement — “paradigmatically ‘a caused b’” (p. 148) — tells us that some specific event a brought about some specific event b, an abstract statement instead posits a causal relationship between events of types A and B “such that, if this relationship unfolds without interference, an A-type event will produce a B-type event” (p. 126). This is the difference between “my dog Zorri did not go on a walk this morning, and this made her unhappy today” and “not going on morning walks makes dogs unhappy.”[3] Distinguishing between the two types of statement is important because it is impossible to provide direct empirical evidence for or against the notion that an abstract causal statement expresses; “not going on morning walks makes dogs unhappy” is not a claim about observable frequency, but about propensity, and as such says not that most dogs who do not get morning walks are unhappy, but that dogs that do not get morning walks are unhappy unless something else intervenes to prevent that from happening. A propensity statement can thus be true even if the evidence is inconclusive, because evidence pertains to concrete explanatory statements,........

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