The psychologist's problem. Since its inception in the 19th century, psychological science has made steady progress investigating perceptual and motoric abilities—how the visual system encodes color, for example, or how we shift our gaze to peripheral events. Much less is understood about abilities thatmake us distinctly human. Some information is available about themechanisms of natural language. But there is hardly any insight into how people create scientific theories about the world. This is an embarassing gap for Psychology since scientific achievement is themost distinctive and remarkable feature of our species.

What's blocking progress is that the most natural account of this ability seems to face an insuperable difficulty. According to the account in question, most everyone has an innate disposition to reason in rough conformity with normatively correct principles of deductive and inductive logic—just as most everyone is endowed with perceptual mechanisms that give us a roughly accurate picture of the environment. How else could our ancestors have met the challenges of survival? It is the twin pillars of natural reasoning—deductive and inductive—that allow people to draw out the consequences of rival scientific theories and assign sensible credibilities to each in the light of data.

These vague remarks are just an attempt to prepare for sharper theories. But we stumble even at this initial step, because one of the twin pillars seems to be absent. The problem is not so much with deductive logic. It can be challenging to communicate the informal concept of logical necessity. But once this is achieved, most people distinguish validity from invalidity on an intuitive basis across a broad class of arguments.