Last night I went to a dinner party. One guest was talking about how different people believe different things and that’s just how it is and that’s fine. According to her, it’s all about diversity, and tolerance, and acceptance, and realizing that different people have every right to form their own opinions.
That sounds unassailably correct until you think harder about it and realize that it’s not. I pointed to a tree outside the window and asked her if she thought it was OK for a person to believe that the tree was not actually there. Somewhat trapped, she asserted that yes, it would indeed be OK. Perhaps it was me that was hallucinating the existence of the tree. Perhaps the person is living in a parallel universe where the tree does not in fact exist. Perhaps the tree is a hologram.
The problem with this line of thought is that beliefs have consequences. If I go out and pick an apple from the “non-existent” tree and bring it into the house and ask the person to take a bite, they will be hard-pressed to deny that they are biting into a real apple. If the “non-existent” tree is blown over by strong winds and crashes into the roof of the house, the damage will be very real no matter what the person believed about the existence of the tree. Or if I am standing on the roof of a building and believe that I can fly and decide to therefore jump off, I will die. If I believe that global warming is not caused by humans, and thus choose to take no action, the Marshall Islands will eventually disappear into the ocean.
Let’s consider the possibility that the person has a neurological deficit which prevents them from recognizing trees. They see the world as we do, but without trees. Or perhaps, they see trees as blobs of colors, or as a telephone pole, or as aliens taking the form of trees. Are they now justified in asserting a belief that the tree outside the window does not exist, and are we to accept this belief as reasonable? Hardly. The tree they do not recognize will still destroy their house if it is blown over on top of it. The tree they do not recognize will still produce apples they can eat. They may believe the tree does not exist, but this belief is incorrect, and this incorrect belief will inevitably have more or less severe negative consequences. They simply have an incorrect belief based on defective mechanisms of perception. We do them no favors by accepting their incorrect belief based on some notion of tolerance or diversity or subjectivity.
But what does it mean to say that the tree exists? In what realm does it exist? How do we “know” that it exists? If 50% of the people in the world have the neurological processing defect rendering them unable to perceive trees, who is to say that they are wrong and the other 50% are right?
Well, we can go out and cut a branch off the tree and bring it in the house and give it to them to hold and ask them, if the tree does not exist, where did the branch come from? We can cut down the tree and examine its tree rings and establish that it is not a telephone pole. We can subject the tree to spectographic analysis and identify the results as being consistent with all trees. We can measure the height of the tree over time and watch it grow. At some point, the sequence of denials and misperceptions necessary to continue to to assert that the tree does not exist will simply grow too unwieldy for anyone other than those completely divorced from reality to continue to support.
But what exactly is this tree that we are so sure exists? At one level, the tree is a construct of our brains. We “see” green leaves, but actually the green in our brains is the result of a long perceptual chain that originates with a certain chemical and atomic structure of the leaves and is then mediated through our visual sensing mechanisms and complex information processing circuits in our brain. We “see” the leaves as a certain shape, but actually that perception of shape is the result of a similar perceptual chain that identifies and groups patterns in our visual input. So the fact that we see green leaf-shaped objects is in some sense a product of our brains, or “minds” if you will. Since everyone has their own minds, at the end of the day, doesn’t this mean that the leaves and tree itself are nothing more than a mental construct, which people therefore should feel free to assert the existence or non-existence of?
This viewpoint fundamentally confuses the question of the brain as creator of reality, vs. the brain as processor of reality. The key point — which must be taken basically as an axiom — is that there is one, single, physical, underlying reality. It is not a reality of trees with green leaves; it is a reality of whirling tangled microcosmoses of atoms and quantum fields. It is our bodies and brains that have developed the ability to capture information about this underlying reality, and process, synthesize, adapt and aggregate it into mental models that are useful, or necessary, to our survival.