The physical object, which he calls the "thing in itself," he regards as essentially unknowable; what can be known is the object as we have it in experience, which he calls the "phenomenon." The phenomenon, being a joint product of us and the thing in itself, is sure to have those characteristics which are due to us, and is therefore sure to conform to our a priori knowledge. Hence this knowledge, though true of all actual and possible experience, must not be supposed to apply outside experience. Thus in spite of the existence of a priori knowledge, we cannot know anything about the thing in itself or about what is not an actual or possible object of experience. In this way he tries to reconcile and harmonise the contentions of the rationalists with the arguments of the empiricists.
Apart from minor grounds on which Kant's philosophy may be criticised, there is one main objection which seems fatal to any attempt to deal with the problem of a priori knowledge by his method. The thing to be accounted for is our certainty that the facts
