To every kind of object, according to Aristotle, there corresponds a proper method of inquiry. It is also well known that, in Kant’s view, neglecting the boundaries between disciplines brings about confusion. In a quite opposite spirit, Maxwell, as it were, encouraged ‘interbreeding’ between sciences, and likened the endeavours of a scientist to the work of a bee who, by mixing up different sorts of pollen, brings about a welcome diversification of flower species.
Nonetheless, in spite of Maxwell’s optimism, it hardly seems sufficient, in order to overcome both Aristotle’s and Kant’s strict methodological stances, to appeal loosely to past, sporadic cases of successful collaboration among sciences, such as molecular biology or neuropsychology for instance. The history of sciences teaches us that, as a matter of fact, Aristotle and Kant have been proved right in nine cases out of ten (think to mathematical psychology (Herbart, Lewin), energetics (Ostwald), psychophysics (Fechner), thermodynamical mechanics (Duhem) etc.
On the other hand, according to the thesis I intend to endorse, metaphysics, once properly construed as an epistemologically responsible and empirically informed theory construction, is the field where interdisciplinary success is most likely to be expected.
An example can illustrate the point. Consider the Müller-Lyer well known optical illusion, featuring two segments of the same length, which nonetheless appear to us as differing in length due to the presence of two little arrow tails at both of their ends.
Prominent scholars reflecting on perception have held that, properly speaking, we should consider real what actually appears to us, rather than the physically measured object. The two segments then, on their view, would actually differ in length (Kanisza). It seems rather plausible, on the other hand, to insist that, insofar as we are dealing with an illusion, the two segments do not differ actually in length.
It is perfectly sensible to inquire into the reason why we normally feel inclined towards favouring the physical account over the psychological one. In particular, B appears to us as shorter than A because our visual system tends to construe it as the closer edge of a prism, while construing A as its inner and further edge. This consideration allows us to put forward the following epistemological principle:
Whenever the claims produced by a physical methodology contradict claims produced by a psychological methodology, we are justified in favouring the former over the latter if we possess a satisfactory naturalistic account of why the world appears to us as different from what it actually is.
The above principle allows for a large number of cases in which we are justified in sticking with physics rather than psychology. Consider the case of the perceptual anisotropy of space, as opposed to its physical isotropy. We distinguish between up and down in virtue of gravity acting upon our labyrinths. Matters do not appear quite as straightforward when we are confronted with more controversial cases, such as Quantum Nonseparability or the Eternalism favoured by special relativity theory. These situations also imply a conflict between physical and psychological accounts, but in these cases we do not possess a satisfactory naturalistic account of why the world appears to us as different from what physics tells us. Accordingly, the second part of our principle, that we might call ‘moderate physicalism’, could be the following:
Whenever the claims produced by a physical methodology contradict claims produced by a psychological methodology, we are justified in favouring the latter over the former if we do not possess a satisfactory naturalistic account of why the world appears to us as different from what it actually is.
We believe that the merits of any interdisciplinary approach will be directly proportional to the quantity of knowledge to which it gives us access. Our thesis is that this knowledge is very likely to stem from an epistemologically responsible philosophical reflection on the relations amongst empirical sciences. The above example of moderate physicalism shows how this kind of machinery could work.