There are two ways to think about “the edge of reason.” Let us start with the most obvious way, the one that has been under attack from an array of unlikely bedfellows in recent years, ranging from evolutionary psychologists to postmodern philosophers. For them, “the edge of reason” gestures menacingly towards what lies on the other side of reason—so-called “unreason,” a realm of error and disorder that results when we stray from the dictates of logic. Here “edge” functions as a boundary concept. It suggests that reason is something that we already possess but can easily lose if we are not careful and unwittingly pass over to the “dark side.” This is the sense of “reason” that was originally the preserve of priests and then psychiatrists—with modernist philosophers kibitzing from the sidelines.
Implied here is the idea that reason—often written “Reason”—stands as an intellectual bulwark against the baser, unreflective, less comprehensive aspects of our mental life. It is just this image of the mind as a house divided against itself that is opposed by those who would deny reason its edge. For better or worse, these critics argue, we respond most authentically to the environments in which we normally find ourselves, and typically we do so without anything that looks like a rational argument. Our existential problems as a culture or even as a species begin once we try to generalise wildly from this limited base of experience—or still worse our rationalisations of that experience. There is literally no “Reason” to think that behaviours that have proven adaptive in one environment will be successful elsewhere.