James Maclaurin

Philosophers differ widely in the extent to which they condone the exploration of the realms of possibilia. Some are very enamoured of thought experiments in which human intuition is trained upon the products of human imagination. Others are much more sceptical of the fruits of such purely cognitive explorations. That said, it is clear that human beings cannot dispense with modal speculation altogether. Rationality rests upon the ability to make decisions and that in turn rests upon the ability to learn about what is possible and what is probable. Thus, on pain of irrationality, we must have some means of exploring other possible worlds. Thankfully, intuition is not the only aid we have at our disposal. Science also is in the business of finding regularities, which hold counterfactually. Scientific theory tells us about the likelihood of particular outcomes flowing from particular processes given particular background conditions. Thus, it also tells us about the contents of 1 of 21 other possible worlds. One consequence of the possibility of such inferences has been a theoretical interest, not just in the contents, but also in the geography of the domain of all possibly worlds. Metaphysicians, epistemologists and philosophers of language are very familiar with locutions such as “nearby possible worlds”. Similarly, evolutionary theory tells us that there is little chance of us discovering an organism that is mammal-like in most respects except in having six limbs. It’s not that we know such an organism to be impossible, but rather that we think it would be the product of an evolutionary history very different to the actual history of life on earth. Put another way, such organisms would be denizens of distant possible worlds. Clearly then, both biology and philosophy have ample motivation to be interested in the reasoning and evidence that supports such claims. Seemingly, in both disciplines there is a certain lure to this modal cartography, but ought we in fact to be convinced of its merits? Is it science or philosophy or not a good example of either? What sort of problems can it solve? What sort of problems will it create? How might we test its accuracy? In his excellent book Theoretical Morphology: The Concept and Its Applications, George McGhee provides an admirable introduction to the complex theoretical landscape surrounding the exploration of possible biological form.