ABSTRACT
Reason, in the sense of reasoning, is of course the main instrument of philosophy. The reasoning that has been offered has been commonly regarded as conclusive or demonstrative, and not of the merely probable kind that must serve in the natural sciences. ‘All daffodils I have seen have been yellow, so the ones I have still to see will probably also be yellow’; refinements apart, the generalizations of natural science all rest on reasoning of this type, and none of them are certain, in the sense that we can see them to be necessarily true. But philosophers have sought certainty, and have commonly believed themselves to have an instrument that would provide it. This instrument is a priori thinking. The philosopher who may have discovered it, Pythagoras, was convinced that he had found the key to the nature of things, and its power seems to have left him intoxicated with wonder and delight. Nor was its use confined to an intellectual elite. Socrates was convinced that the faculty for it was present in all men, even in the untutored slave boy of the Meno, and that the education of this faculty was the supreme requisite not only for the philosopher, but also for the statesman. In documents that provided the first large-scale exhibition of it, the Platonic dialogues, Plato undertook to show how the new tool could be used both to direct conduct and to give insight into the permanent structure of the world.
