So far the philosophical thickets in which I have rummaged have been thickets that have grown up because of boundary disputes between theories or views which were not themselves philosophers’ theories or views. The litigations between the disputants were, necessarily, philosophical troubles, but the original disputants were, for example, mathematicians and men in the street, physiologists and landscape painters, or psychologists and moral instructors.

But now I want to discuss a domestic issue which has fairly recently broken out between certain philosophers and certain philosophically-minded logicians. I shall not do more than give an outline sketch of the situation, since I want to conclude by characterizing against this outline some pervasive features of the variegated thickets in which I have been rummaging.

Since Aristotle, there has existed a branch of inquiries, often entitled ‘Formal Logic’, which has always adhered more or less closely to general philosophical inquiries. It is not easy to describe this liaison between Formal Logic and philosophy. The systematic presentation of the rules of syllogistic inference is a very different sort of activity from, say, the elucidation of the concept of pleasure. The Aristotle who inaugurated the former is the same thinker as the Aristotle who considerably developed the latter, yet the kinds of thinking in which he was involved are very widely different. The technical problems in the theory of the syllogism have a strong resemblance to the problems of Euclidean geometry; the ideals of systematization and rigorous proof are at work, questions of switches and shades of significance are barred, false moves are demonstrable fallacies. The problems in, say, the theory of pleasure or perception or moral responsibility are not like this. Aristotle debates with Plato and Socrates, and the issues become better defined as the debate progresses, but the debate does not take the shape of a chain of theorems, nor do the arguments used in that debate admit of notational codification. Whether a given philosophical argument is valid or fallacious is, in general, itself a debatable question. Simple inspection cannot decide. More often it is a question of whether the argument has much, little or no force. Yet different though Formal Logic is from philosophy, the operations characteristic of Formal Logic exercise a detectable, if minor, control over the operations characteristic of philosophy.