Harvard's Emerson Hall was the home of both the philosophy department and the psychology laboratories in the final years of Williams James's eclectic, discipline-crossing career. In Francesca Bordogna's new book, the building becomes a metaphor for James's purposeful trespassing of boundaries: between empirical psychology and introspective philosophy, between mainstream psychology and parapsychology, and between amateur and professional science.

For James, psychology and philosophy needed each other, as Bordogna nicely brings out in her discussion of James's review of a book by the neurophysiologist David Ferrier. Documenting Ferrier's epoch-making experiments stimulating and destroying parts of animal brains and observing associated changes in muscular activity, the book reported interestingly different results for dogs and monkeys. In general Ferrier expected stimulation to induce activity and destruction to inhibit it. So he found with monkeys. But while the predicted stimulation effects were observed in dogs, the inhibition effects were not; sometimes the destruction of particular cortical areas in dogs even led to better control of the associated muscle. Ferrier's explanation was that muscular activity in monkeys ‘could only be performed under the direct control of the will’, but in dogs ‘the same movements could also occur in an automatic way’. In James's review, he noted that Ferrier here relied tacitly on ‘an appeal to introspective philosophy’ (p. 73).

Bordogna argues that James saw in psychical research a chance to study the complications of the mind but also to challenge the rigid boundary between mainstream science and amateur research into the abnormal. One common objection to reports of psychic phenomena was that the probability that witnesses were dishonest was higher than the probability that the phenomena had really occurred. The ‘faggot’ argument, due to James's friend Edmund Gurney, was meant to counter this objection. A faggot is a bundle of woven sticks; the eponymous argument is that, as more independent witnesses observe a phenomenon, the probability that they all cheated decreases to below the probability that the phenomenon was real. (Readers might recognize this as a robustness argument.) Bordogna describes disputants' attitudes towards the argument. James thought it ‘carried heavy weight’ (p. 121), and that the ‘various fragments of evidence’ regarding a psychic phenomenon ‘resembled the recording of a multivocal performance’. For James, ‘weak sticks make strong faggots’ (p. 133). In response, a critic argued that ‘when we have an enormous number of cases, and cannot find among them all a single one that is quite conclusive, the very number of cases may be interpreted as an index of the weakness of the evidence’ (p. 121). Notoriously, James even came to argue that, in Bordogna's words, ‘it was permissible to accept a belief in the absence of sufficient evidence’ (p. 117). In Ian Hacking's 1988 paper on psychical research of the same period (Isis 79, pp. 427–451), he argued that the experimental innovation of randomization raised the quality of psychical research, which led to the debunking of psychical claims. Bordogna does not cite Hacking, but the tension between the evidential force of all available evidence versus the evidential force of only the best available evidence would make for an interesting discussion: when faggots of evidence and gold-standard evidence contradict each other, which should we believe?

The troubled border between normative and descriptive approaches to philosophy is another of the boundaries that James negotiated throughout his career. Bordogna devotes two chapters to analysing the controversy about emergent American pragmatism ‘as a clash between two different visions of the future of philosophy as a discipline’ (p. 139). Competing theories of truth – logical and psychological, normative and descriptive – defined these two visions. The descriptive approach with which James came to be identified stressed the physiological and psychological aspects of human cognition. For example, psychologists were interested in the feelings that agreement or disagreement between ideas produced – that ‘in harmony or discord itself there is something immediately satisfying or painful’ (p. 150). Philosophical critics, however, claimed that psychological processes are distinct from the logic of truth; to emphasize the former was merely ‘psychologism’, and philosophers should be concerned only with analysis of what truth is and how it can reliably be obtained. But what Bordogna calls ‘embodied truth’ and the ‘psychology of truth’ were important avenues of research for James's pragmatism.

Bordogna's final topics are the porous boundaries of the self and of disciplines. On the former, she shows that, for James, the fragmented self ‘was instrumental to rooting the self in community and to promoting new kinds of human relationships’ (p. 208). On the latter, she offers a fascinating discussion of ‘trees of knowledge’ – diagrams of the relations between disciplines. Such visual classifications of knowledge, which go back at least to Bacon (and probably Aristotle), are usually meant to help unify disparate disciplines, but are also used as power plays in disciplinary politics. Some of James's colleagues placed philosophy at the top of their trees (or at the centres of circles). James would have none of that. In Bordogna's words, philosophy for James was ‘a form of mediation between diverse modes of inquiry’ (p. 245).

Bordogna masterfully meets a self-reflexive challenge: she has taken an important figure in the history of philosophy and psychology who deliberately blurred boundaries and written about him from the perspectives of a historian of philosophy, a historian of science and a philosopher of science. At the same time, she commits occasional disciplinary vices. For example, she has the philosopher's habit of not indicating publication dates of quoted passages. You might think that at least the footnotes would tell you these dates, but no – one must first flip to the footnotes, and then to the bibliography, of which there are three: for James, for other primary sources and for secondary literature. Trifles aside, however, this is an excellent book. I recommend it to historians of psychology and parapsychology, historians of fin de siècle philosophy, and anyone interested in the history of philosophy of science.