When, in the eighteenth century, David Hume proposed that it is crucial for people to believe in what no reasoning or evidence can lead them to believe, he could not surmise that in the twentieth century this proposal would become the embryo of a widely endorsed naturalist project, which has come to express itself in the view that there is nothing to be said about knowledge except what can result from an investigation on the formation of beliefs, whether it be of a sociological, psychological, or biological character.