Scepticus: I have a question for you, Olive. Are the only things about which we can have certain knowledge are the contents of one’s own mind? Is it possible to have knowledge of the external world?
Olive: Interesting. I will approach this question with another question – is there anything that we can be 100% sure that exists only in our mind and does not exist in the external world?
Scepticus: You tell me.
Olive: My initial reaction is to say no. It seems to me that everything we think about, even our dreams, is connected to the external world. What if I think about a dog? I know of dogs through my experience of them – seeing them, touching them, smelling them, etc. This is obvious.
What if I make up something? Imagine an object that I will call… a needge. It is green, solid, and travels through walls when I push it with two fingers. Even though this needge is completely fictional, it stems from my experience of the world. My experience of green, walls, movement, fingers… Now I have never experienced a solid object pass through a wall. Is this idea in my mind and not in the external world?
Scepticus: Why, yes it is!
Olive: But if I had no knowledge of walls, movement, what it means for an object to pass through another, etc, would I be able to conceive of an object passing through a wall?
Scepticus: Well, perhaps not. But does the needge’s origin is in the external world matter? Can’t we just say that the needge’s ability to pass through walls is in your mind and leave it at that?
Olive: Think about it this way – imagine that you never had any sensation. From the moment of conception, you were completely deprived of any sensory input. Could you conceive a needge? Could you even think at all?
Scepticus: pauses. No, you couldn’t conceive of a needge. But this doesn’t mean that you wouldn’t think. I just don’t know what you would think of…
Olive: Sensation is the conduit for thinking, Scepticus! No matter how much you explain to a child who is blind, and has been so for her whole life, about the redness of an apple, she will not understand it! She cannot even imagine it.
Scepticus: A blind child, sure, but what about a man who lost his hand in an accident? I read that people who lose limbs experience phantom sensations. Obviously, there is no object-prompted sensation. So phantom sensation must exist in the mind!
Olive: True, these sensations do exist in the mind, but again I ask of the poor person deprived of any sense experience. Would he have phantom sensations?
Indulge me for a moment, while I approach the issue of knowing the external world from a different angle. What if, when you sense something, you are not necessarily sensing the “object”, but rather your experience is that of your senses. You do not experience anything other than your senses. Therefore, everything you experience is contained within your mind.
Scepticus: What about the object that prompts sensation?
Olive: I liken this to a pool of water. Suppose that you are standing in a shallow pool. A few feet from where you stand, a pebble falls into the water. This creates ripples, which after a moment splash against your legs. You experience the rock through the ripple’s effect. This is like sensation. The object that you sense through the ripple is the rock. Your experience of the rock is the ripple. Therefore, your knowledge that the rock has fallen into the pool is indirect.
Scepticus: You are saying that we cannot have knowledge of the external world?
Olive: We rely on the external world for experience. We can have knowledge of the external world, but it is always within the context of our own bodies and minds. Essentially, I am creating a distinction between sensation and the object. In common speech, when I say “This snow feels cold” we interpret that to mean “The snow is cold.” What I’m saying in reply to “This snow feels cold” is “My experience of my sensation is cold.”
Is it possible to have a ripple without a pebble? I think that this question is closely related to your question about phantom sensations. A phantom sensation would be a ripple without a pebble. But in real life I think that it is more nuanced that this. The only way you can have a phantom sensation is because you used to have a limb.
So, do we have knowledge of the external world? I am answering with both yes and no. Yes, because without the external world I doubt we could think at all. No, because our knowledge is indirect. We only know the "contents" of our body.
