The associations of the word “impression” are not to be mistaken.  Locke had taught that between ideas in the memory and genuine sensations there is the difference that the latter are due to the “brisk acting” of objects without us.  Objects impress us, and we have sensations or impressions.  To be sure, Hume, after employing the word “impression,” goes on to argue that we have no evidence that there are external objects, which cause impressions.  But he retains the word “impression,” nevertheless, and his use of it perceptibly colors his thought.

In Kant’s distinction between phenomena and noumena we have the lineal descendant of the old distinction between the circle of our ideas and the something outside of them that causes them and of which they are supposed to give information.  Hume said we have no reason to believe such a thing exists, but are impelled by our nature to believe in it.  Kant is not so much concerned to prove the nonexistence of noumena, things-in-themselves, as he is to prove that the very conception is an empty one.  His reasonings seem to result in the conclusion that we can make no intelligible statement about things so cut off from our experience as noumena are supposed to be; and one would imagine that he would have felt impelled to go on to the frank declaration that we have no reason to believe in noumena at all, and had better throw away altogether so meaningless and useless a notion.  But he was a conservative creature, and he did not go quite so far.

So far there is little choice between Kant and Hume.  Certainly the former does not appear to have rehabilitated the external world which had suffered from the assaults of his predecessors.  What important difference is there between his doctrine and that of the man whose skeptical tendencies he wished to combat?

The difference is this:  Descartes and Locke had accounted for our knowledge of things by maintaining that things act upon us, and make an impression or sensation—­that their action, so to speak, begets ideas.  This is a very ancient doctrine as well as a very modern one; it is the doctrine that most men find reasonable even before they devote themselves to the study of philosophy.  The totality of such impressions received from the external world, they are accustomed to regard as our experience of external things; and they are inclined to think that any knowledge of external things not founded upon experience can hardly deserve the name of knowledge.

Now, Hume, when he cast doubt upon the existence of external things, did not, as I have said above, divest himself of the suggestions of the word “impression.”  He insists strenuously that all our knowledge is founded upon experience; and he holds that no experience can give us knowledge that is necessary and universal.  We know things as they are revealed to us in our experience; but who can guarantee that we may not have new experiences of a quite different kind, and which flatly contradict the notions which we have so far attained of what is possible and impossible, true and untrue.