Questions about what matters, and why, and what exists in the world, are quintessentially philosophical. The answers to many of these questions are informed by how we conceive of ourselves. How has what is often described as the ‘Copernican revolution’ effected by Charles Darwin changed our self-conception? One particularly surprising feature of evolutionary biology is that it lends significant support to existentialism.
To make the journey from evolutionary biology to existentialism, let’s start with one of the oldest and most profound of philosophical questions: how do we decide what is right or wrong, good or bad? Many philosophers have warned that no facts about nature can ever provide grounds for claims about values. In A Treatise of Human Nature (1739-40), David Hume made a compelling argument that an unbridgeable gap separates fact from value, ‘is’ from ‘ought’. To attempt to bridge that gap is to commit the ‘naturalistic fallacy’: no argument from factual premises can ever secure a conclusion about what is valuable, or about what to do.
In Principia Ethica (1903), the philosopher G E Moore adduced what he called an ‘open question argument’ for a similar constraint on any discourse about value. He claimed that any attempt to define ‘good’ in naturalistic terms – such as pleasure, or ‘utility’, or for that matter a divine commandment – must fail, because it always remains an ‘open question’ whether pleasure, or utility, or that particular divine commandment, really is good. If such a definition were sound, like the definition of a triangle as a three-angled plane figure, then the question would not make sense. ‘Is a triangle really a three-sided figure?’ would merely signal a failure to understand the definition.
Moore’s formulation has given rise to an enormous amount of debate. It is reasonable to object that it begs the question: for if one of those definitions of ‘good’ is correct, then the question is no longer open. But it must be admitted that no definition of ‘good’ in terms of some natural properties is very plausible. Hume’s original ‘is-ought’ gap still gapes.