Nothing exists. Even if something does exist, it cannot be grasped. Even if it can be grasped, it cannot be expressed.
— Gorgias
I think, therefore I am.
— René Descartes
The world I perceive is not enough to prove that the world truly exists. But the fact that there is something capable of perceiving it does at least point to the existence of a thinking self.
What we call sight already shows how fragile our certainty is. Light emitted or reflected by objects is received by the eyes, sent through the optic nerve, and processed by the brain into the image we experience. In other words, what we "see" is not the light itself, but the brain's reconstruction of signals.
That has an unsettling implication: even if the light were absent, or the eyes themselves did not exist, as long as the same neural signals could be fed into the brain, the same image could still appear before us. If perception can be simulated at the level of the nervous system, then we can never be completely sure whether the things we take for granted as external objects truly exist in the way we imagine them.
Idol of the Tribe: it is a false assertion that the human senses are the measure of things. On the contrary, all perceptions, both of the senses and of the mind, are according to the measure of the individual, and not according to the measure of the universe.
— Francis Bacon
Human knowledge comes from summarizing and generalizing experience. That knowledge may help us live more effectively, but usefulness is not the same as truth. The world we experience is not the world in itself—at the very least, it is not the whole of it. It is only the collection of what our senses allow us to register.
The eyes receive light. The ears receive sound waves produced by vibration. Nerves distributed across the body register temperature and the pressure of contact on the skin. The nose and tongue respond to chemical stimulation carried by odor and taste molecules. All of this information is then transmitted to the brain, where it becomes the images we see, the sounds we hear, the textures we feel, and the smells and flavors we recognize.
Yet visible light is only a tiny fragment of the electromagnetic spectrum. Most electromagnetic waves are entirely inaccessible to unaided human vision. More broadly, nearly all the information available in the outside world lies beyond what human beings can directly detect. The amount of information we can obtain is severely restricted, and the very dimensions in which we can obtain it are restricted as well.
We know that light exists and that it is an electromagnetic wave, so through science we have learned to detect forms of electromagnetic radiation that we cannot see. But there may be kinds of things for which we possess no sensory entry point at all. What we cannot sense, we struggle even to conceive; what we cannot conceive, we can hardly describe in language.
For a person born blind, two objects that are identical in every respect except color are not different in any meaningful way. And it is extremely difficult to explain what color is to someone who has never had visual experience. If all human beings were born blind, the word "color" and the concept behind it would never have arisen at all.
Fortunately, most humans do have eyes. Unfortunately, we have no idea in which dimensions all of humanity may be "born blind."
Modern science and technology let us reach beyond our native senses. We can infer the existence of gravitational waves, track planetary motion, and observe the expansion of the universe. But even this expansion of knowledge remains limited. Everything we know is still built on the structure of human thought. Just as a person born blind does not naturally arrive at the concept of color, we too may lack the concepts needed for whole regions of reality. Even with scientific instruments, what we can know may still be no more than the tip of an iceberg.
Our perceptual world is so narrow, and yet we arrogantly call it reality. That is the truly absurd part.