From the Mereological Fallacy to the Metonymical Trap

Maxwell Bennett and Peter Hacker (2023) famously introduced the so-called charge of ‘mereological fallacy’. This fallacy is not per se a reasoning fallacy but is more akin to an attribution mistake (an attribution mistake that can admittedly lead to fallacies of reasoning). In the context that is of particular interest to Bennett and Hacker's discussion, the ‘mereological fallacy’ is specifically related to neuroscience: to say, for example as part of a neuroscientific explanation, that the brain constructs hypotheses would be a paradigmatic instance of a mereological fallacy (ibid, 80). A brain is indeed nothing more than a part of a human being, and only of a whole human being does it make sense to say that it constructs hypotheses. Therefore, transferring the qualities and capacities of the whole (the human being) to the part (its brain) would not be a viable philosophical option. It would not be an option in particular because it is partly constitutive of the meaning of the attributed expression (‘to construct hypotheses’) that a range of behaviour be associated with it, a range of behaviour that a brain obviously lacks. This idea is Wittgensteinian through and through, and found in the oft-quoted passage of the Philosophical Investigations (Wittgenstein 2009: PI §103): ‘It comes to this: only of a living human being and what resembles (behaves like) a living human being can one say: it has sensations; it sees; is blind; hears; is deaf; is conscious or unconscious’. A brain does not – in any way, shape or form – behave like a living human being.

Returning to the more general diagnosis of mereological fallacy: although there are numerous instances of predicates that can equally and harmlessly be attributed indistinctly to a whole or to a part of that whole (the fallacy is not systematic) and even if mereological fallacies can apply in a variety of contexts (to borrow one of Hacker's examples (2013, 287) a clock indicates time, a capacity that its fusee or face could not be said to possess), we can say that such a diagnosis is particularly fruitful when it concerns the so-called psycho-logical predicates. These predicates relate to the mental life and the cognitive or psychological capacities of a person.