This is a continuation of my series On How We Know. The introduction can be found here, and a full listing of articles can be found here.
In the book A Christmas Carol by Charles Dickens, the ghost of Jacob Marley confronts the character Ebenezer Scrooge, and after introducing himself the ghost and Scrooge have a rather interesting conversation. I will include the first few lines here:
"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 own 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!"
In this exchange the ghost of Jacob Marley asks a rather interesting question, "What evidence would you have of my reality beyond that of your own senses?" Essentially Jacob Marley is asking, "Is there any other possible way for you, or anyone else, to gain knowledge of reality than through your senses?" and the natural conclusion to this thought is that if there is no other way to gain knowledge of reality then, "Why do you doubt your senses?"
Scrooge's response is anything but unique. It is the response of sceptics and philosophers from many ages of Western Philosophy. Perhaps the one who expressed it better than anyone else, and is best known for it, is Rene Descartes. His particular approach even has its own name, the method of doubt. In applying the method of doubt, Descartes had a specific goal in mind, to find the foundations of reality, but unintentionally his method of doubt started a tradition that has continued and influenced us to this day. It introduced into not just Western Philosophy, but all of Western Culture a fundamental distrust of our senses. I have heard Descartes', and Scrooge's, arguments repeated over and over by college professors, by high school teachers and even by elementary school children. Even though most people live and act as if their senses are a good indication of what is real, the same people will immediate express doubt in their own senses, and especially those of others, when what they sense does not agree with what they already "know". There is always some other explanation that explains what someone saw or otherwise sensed. These doubts are given particular force because of the tradition we have of doubting or senses.
So let us consider this doubt, do we have any reason at all to doubt our senses? Essentially the argument of Descartes, and Scrooge, is that there exist well known instances where our senses cannot be relied upon. The classic example is dreams. We can "sense" things in our dreams that are not really there. This is not confined to our dreams but occasionally, as Scrooge points out, our senses can be cheated by other things, such as indigestion or other substances. So the question is, "Where do we draw the line?" How do we determine which sensations are real and which are false? The answer of Descartes was essentially, "Because I don't know where the line is I am going to assume that there is no line and that I can't trust any of the sensations I have." While Descartes ultimately acknowledges that this approach is insane, the damage was done and he had introduced the idea that because there is some doubt as to where the dividing line is between two things, in this case sensations corresponding to reality and those that do not, then we cannot assume that any of our sensations correspond to reality and we must doubt everything.
Effectively what Descartes has done is to confuse the ocean for a continent because he was standing on the beach and did not know where "dry land" ended and the water began. The method used by Descartes and the argument of Scrooge is like someone standing on a beach and wondering where the land ends and where the water begins. After puzzling over it for a while this person concludes that there is no such thing as land because they cannot clearly discern the boundary between a continent and an ocean. They then head inland away from the ocean and begin to talk to people and tell them that they are not standing on dry land but that they are actually in the middle of the ocean. In their defence they point to the presence of lakes, rivers and even glasses of water to prove that we do not live on a continent but that we live in the ocean. If someone actually attempted this and spoke like this then they would quickly be picked up by the authorities and taken to a "safe place" with padded walls. But in the case of philosophers instead of being ignored as insane they are called great and have their works and ideas spread around like manure.
Just because the boundary between an ocean and a continent is not well defined to someone standing on the beach does not mean that there is not a distinct, well definable and immediately recognizable difference between the two. Doubting the existence of a continent because the waves are washing your feet is an act of severe intellectual dishonesty. In the same way, doubting all our senses because a few of them may not actually correspond to anything in reality is also an act of severe intellectual dishonesty.
Continuing with this analogy we note that on continents there are lake and rivers which some will use to cast doubt on our assertion. These I will liken to physical injury, drugs and other mind altering substances. The fact that I can mention them in the context of "mind altering" indicates that there is a distinct, well definable and immediately recognizable difference between normal sensations and those arising from drugs or physical conditions. Again there is no reason to doubt the veracity of all sensations, or the existence of a continent, just because you are dabbling your feet in a river or a lake.
Related to this is the misguided approach of trying to "discover" something about reality by deliberately partaking in mind altering drugs. This would be akin to stating a desire to understand the rocks and dirt of a continent and then promptly going for a swim and spending your time staring at a fish.
So how do know to distinguish between between sensations that give us knowledge of reality and those that do not? The answer to that question is inextricably bound up with the answer to the question "How do we gain knowledge?" which is the purpose of these essays. So we know the limit or boundary by applying the selfsame modes of knowing that lead us into all knowledge. As for giving a more exact answer I would ask, "How do we recognize the boundary between land and water?" Understand that and you can learn to recognize the boundaries between sensations that give us knowledge of reality and those that do not.