“You don’t believe in me,” observed the Ghost.
“I don’t,” said Scrooge.
“What evidence would you have of my reality beyond that of your senses?” “I don’t know,” said Scrooge.
“Why do you doubt your senses?”“Because,” said Scrooge, “a little thing affects them. A slight disorder of the stomach makes them cheats. You may be an undigested bit of beef, a blot of mustard, a crumb of cheese, a fragment of an underdone potato. There’s more of gravy than of grave about you, whatever you are!”

—Charles Dickens, A Christmas Carol

In this passage the ghost of Scrooge’s former partner, Jacob Marley, returns to lead him back to the “right way” in life.  There is, however, an initial problem: Scrooge will not accept the evidence of his senses and admit that the ghost he is seeing is real. Hume begins with a similar criticism of sense in the Enquiry on Human Understanding: 

I need not insist upon the more trite topics, employed by the sceptics in all ages, against the evidence of sense; such as those which are derived from the imperfection and fallaciousness of our organs, on numberless occasions; the crooked appearance of an oar in water; the various aspects of objects, according to their different distances; the double images which arise from the pressing one eye; with many other appearances of a like nature.

Thus, like Scrooge he points out the imperfections of sensation arising either on the side of the organ or that of the situation of the object in respect to the organ we find in daily life. He does not, however, insist upon this since he realizes that we habitually correct for these defects; rather, he continues to point out a more fundamental defect in the senses: sensation is of images, not things:

But this universal and primary opinion of all men is soon destroyed by the slightest philosophy, which teaches us, that nothing can ever be present to the mind but an image or perception, and that the senses are only the inlets, through which these images are conveyed, without being able to produce any immediate intercourse between the mind and the object. The table, which we see, seems to diminish, as we remove farther from it: but the real table, which exists independent of us, suffers no alteration: it was, therefore, nothing but its image, which was present to the mind. These are the obvious dictates of reason; and no man, who reflects, ever doubted, that the existences, which we consider, when we say, this house and that tree, are nothing but perceptions in the mind, and fleeting copies or representations of other existences, which remain uniform and independent…

It is a question of fact, whether the perceptions of the senses be produced by external objects, resembling them: how shall this question be determined? By experience surely; as all other questions of a like nature. But here experience is, and must be entirely silent. The mind has never anything present to it but the perceptions, and cannot possibly reach any experience of their connexion with objects. The supposition of such a connexion is, therefore, without any foundation in reasoning.  (An Enquiry Concerning Human Understandingemphasis my own)

Since sensation is of images, Hume argues, there is no immediate intercourse between the mind and its object. But, if all we perceive are images, how can we know that there exists some thing corresponding to the images?

There are two main defects with this argument: he admits they are images, what is an image to which nothing real corresponds? Additionally, he has inherited from Locke the view that sensation is becoming similar to the sense. But, before we argue those two points in detail, let us return to see how the ghost deals with Scrooge’s doubts:

“You see this toothpick?”  said Scrooge…

“Well!” returned Scrooge, “I have but to swallow this, and be for the rest of my days persecuted by a legion of goblins, all of my own creation. Humbug, I tell you! humbug!”

At this the spirit raised a frightful cry, and shook its chain with such a dismal and appalling noise, that Scrooge held on tight to his chair, to save himself from falling in a swoon. But how much greater was his horror, when the phantom taking off the bandage round its head, as if it were too warm to wear indoors, its lower jaw dropped down upon its breast!

Scrooge fell upon his knees, and clasped his hands before his face.

“Mercy!” he said. “Dreadful apparition, why do you trouble me?”

“Man of the worldly mind!” replied the Ghost, “do you believe in me or not?”

“I do,” said Scrooge. “I must. But why do spirits walk the earth, and why do they come to me?”

It seems important here that Scrooge was not convinced by an argument. Rather, the ghost scares him, and Scrooge believes.  This seems an important lesson: substance is sensed and this is self-evident or, to put it differently, sense itself provides the evidence for the proposition “sensation is of substance.”

So far I have only contradicted Hume and suggested the problems with his mode of thought, now I will develop those problems into a more formal rebuttal.  The first (and weaker) argument is that Hume seems to admit that the mind receives images. What, then, is an image if not a likeness of something other? If this is what an image is, then my attention to the image as an image is an attention to something besides the image.

It would be odd to deny this fundamental account of an image: saying this would be like saying that a photo or painting does not represent something besides itself. The photo is cherished precisely because in it something other is seen; similarly, we delight in sensation precisely because in them we know another thing.

Hume is at least right in this: this question can only be determined by experience. Since, according to the account I’m proposing, this truth is self-evident, no argument can conclude to it of necessity. He is wrong, however, in saying experience is silent about it: ‘gut instinct’ and passions are all part of our experience and they testify to our conception of the sensible thing as ‘other.’

The other point is the view that sense becomes “similar” to the thing sensed.  This view, as Locke develops it, is a reaction to the idea of innate species. This view, advanced by Descartes, claimed that our knowledge of external things comes from concepts present to our mind by nature: thus, one could in theory derive all the truths of mathematics and physics by merely contemplating in one’s room without doing any experiments.  Seeing the manifest absurdity of this take on sensible reality, Locke went to the other extreme: nothing is known that is not sensed. He then committed the grievous mistake of dividing the powers of natural things to impress themselves upon sense from the impressions upon sense themselves.  Once this is done, since there are two things, the sense impression and the power impressing, one can ask about the correspondence between them.  This question, however, cannot be answered since there is no experience of the correspondence but only of the impression; consequent to this division, Berkeley leads us to deny the reality of external substances.

But, what if this division is denied? Will not Berkeley and Hume’s critique of external substances be nipped in the bud?  To ask whether sense impressions correspond to things becomes meaningless because the sense impression is the unification of the sense power and the thing sensed, the two become, as it were, “one flesh.”  This view traces back to Aristotle’s observations on the unity between agent and patient: just as teaching is not a different activity in re from learning but merely differs in account (i.e. teaching is imparting knowledge to another; learning is receiving knowledge from another) and, similarly, hitting someone else is not a different activity from the other being hit except in account, so the sensible impressing itself upon the sense is not another an activity distinct in re from the sense’s reception of the sensible.

The consequence of all this is that the evidence for sensation being of another is sensation itself.  Since we cannot control whether we sense or not (except per accidens by closing our eyes, etc.) it is manifest that sense is receiving from some external thing.  Since receiving and impressing are one activity, sensation is not merely becoming similar to  an external object, but rather becoming one with such an object.

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