This article explores the possibility that the tendency to believe in an objective, mind-independent external world traces to innate aspects of the human mind. The aspects of mind in question are, first, the capacity to distinguish mental states that have objective referents from those that do not (e.g., perceptual states versus mental imagery), and second, the capacity to mentally represent the continued existence of parts of the world that are beyond the reach of the senses. It is proposed that the evolutionary function of these cognitive abilities relates to the production of novel but adaptive voluntary behaviour. Evidence and arguments are provided in support of the innateness hypothesis. Among these is a Chomskyan-style poverty-of-the-stimulus argument derived from the philosophical literature. The evolutionary account of the subjective-objective distinction leads to the prediction that, in conditions of uncertainty, people will tend to err on the side of assuming the objectivity of their perceptions and other judgements.