In the eighteenth century Immanuel Kant, awakened from his ‘dogmatic slumber’ by his reading of Hume, devised the theory that our perceptions and concepts do not arise willy-nilly and from their own nature but are made to conform to certain innate a priori principles of the mind. The contents of our perceptual fields are laid out in a certain order in space and time not because that part of the world which lies outwith the mind is so constituted but because our minds supply the order and the a priori intuitions of space and time; and we believe that every event has a cause not because every, or even any, event does lie in such a relation but because the very constitution of our minds constrains us to think in terms of the category of causality.
Whatever were Kant’s intentions, such a theory leads directly to scepticism, to the view of knowledge as mere conformity in innate prejudice. Nevertheless, Kant’s view has received support from modern biology at least to the extent that there is evidence—both from direct physiological investigation and general evolutionary principles—that the brain is not a passive receptacle for, say, our sense impressions (which might then be said in some way to ‘correspond’ to external objects) but works them up in accordance with certain principles of its own. As with perception so with the intellect. The prerequisite for evolutionary success, with nervous systems as with any other organ, is survival value. This means, as Henri Bergson among others has maintained, that the evolutionary test of our concepts is the actions they make possible; and this in itself makes plausible the view that our brains, however pliable they may be, in the end constrain us to think in certain ways rather than others with no guarantee that how we think bears any relation at all to the truth. Harry L. Jerison has tried to make a virtue of this sceptical position without, I think, realising its