Suppose you’re a good honest physicalist, you believe that all that exists is physical. This is in stark contrast to philosophies (ontologies) like Berkeley’s idealism, where all things are ideas, reality itself is a form of thought, and therefore the universe is mental or spiritual. The latter idea as a metaphysics has largely fallen out of fashion among philosophers, theologians and scientists alike in the last 100 years since Bertrand Russell’s and G.E. Moore’s attacks on the subject. However there is another form of idealism called epistemological idealism, the view that the contents of human knowledge are inescapably determined by the structure of human consciousness, which may seem radical to some but can be shown to be perfectly sound in a physicalist world through empiricism. We’ll begin with the understanding that human consciousness is experiencing the external world (we’re taking a physicalist stance to start) as a combination of sensation, and from this premise we’ll reach the conclusion of epistemic idealism.

The last century of scientific progress has been unparalleled in the history of mankind, with revolutions happening in nearly every field of science. One of the most important takeaways from many of these advancements, I would argue, is that mankind, or anything for that matter from insects to cameras, cannot be a purely objective observer of their own environment. We cannot stand back and say “This is it! This is what reality really is!”, without taking ourselves into the equation. We can approximately do it, but not to the extent many of us wish. An esoteric example of this inability to be a bystander, is the case of the electron double slit experiment. You get very strange behaviour [left] while shooting electrons between two slits to a detection screen. This made scientists curious about what was going on, so they decided to take a closer look. However when they attempted to check which slit the electron went through, they found entirely different behaviour at the detection screen [right]. The very act of measuring certain things changes their outcomes, because one cannot measure something without interacting with it. We do not think about this regularly because the effects of measurement are often minute on our everyday scale of life. This is true for thermometers as well: if your thermometer is not already the temperature of what you want to measure, heat needs to be transferred between the thermometer and what it is measuring, which changes the temperature of what you’re measuring. An example from zoology is that studying animals in zoo’s or captivity leads us to misunderstand them. More and more studies have shown us that wolves do not have an alpha-beta social hierarchy, this is a false belief spread by misinterpretation of wolf behaviour in captivity. So while we did learn this by observing wolves farther and farther away, with less interference, the lesson is that knowing the external world, implies knowing our place in it as well.

Another much more important piece of this that we’ve begun to understand in the last hundred years or so, is that the kind of world we experience is relative to the structure of the sense organ involved. We simply cannot see infra-red light, x-ray light, ultra-violet light, etc. “But we’ve made fancy tools to detect them!” you might argue. This is true, but these don’t help us experience these forms of light. If you want to see an x-ray, it is displayed using white light. If you want to see in infrared, you wear goggles that shift infra-red to frequencies you can experience. This technique is also true with ultra-high and low frequency sounds. This alone tells us that there is potentially far more to the external world than we can possibly begin to experience (this idea deserves a post of its own), and also leads us into an interesting discussion about the nature of colour, which I will come back to; but first, a cliché. “If a tree falls in the forest, and there is no one around to hear it, does it make a sound?” To tackle this I’ll ask a similar question: What colour is a mirror? This is clearly a question that has a very physical basis but not a very easy observer-independent answer (if this makes you uncomfortable, remember that the velocity of an object is an entirely relational, yet objective property of that object). The answer depends on where I ask the question from. A mirror’s colour is not an inherent property of the mirror, its a relational property to its environment. You cannot surely say it is one colour or another, without a long declaration as to where you are and where the mirror is. The colour of a mirror is a relationship between you, the mirror, and the environment, because the answer depends on how you see color, how the mirror reflects light, and where you and the mirror are relative to each other and the environment. So when we return to the original question, we know the answer: sound is a relationship too. A falling tree makes wiggles in the air, and those wiggles make a sound if and only if an ear is there to have its eardrum and inner ear bones vibrated. Sound is not vibrations in the air, it is a relationship between a listener, air wiggles, and eardrums. In the same vein, you don’t observe wavelengths or frequencies. You see colour, you experience colour. This is why you can not explain what red is like to someone who is blind or has extreme protanomaly (inability to see red light), even if they understand everything they possibly can about light (see: Mary’s room).

Lastly, as far as physicalism is concerned, we know that your brain creates the world you see while you’re experiencing it; it does not purely observe the external world. A simple example of this is when an image hits your eye as a stream of light, the image is inverted when it hits your optic nerve, and so your optic nerve observes the entire world upside-down, simply because of the physics of light and lenses. Your brain however, somehow flips this image so that we do not observe the world vertically flipped. This alone is enough evidence to believe indirect realism (that you do not necessarily experience and observe reality directly), but if you’re unconvinced we also know the brain will fill in visual blind spots with the colours around the blind spot, and the brain also fills in visual blur a la the stopped clock illusion. These are just some examples of many sensory-crafting illusions your brain performs all the time.

All of this may seem disheartening to some who think science should be done with no reference to scientists or humans, and may even find thinking this way to be a violation of the Copernican principle (that humans are not privileged observers of the universe). However I would argue after careful inspection that this understanding is incredibly Copernican, as it indeed addresses that we are not any more special than any other observer. We do not have some perfect ability to pull back the curtain of reality like many of us assume, and have assumed for centuries. In acknowledging our limits and understanding our relationship with the external world, we are lead to a better and more honest understanding of reality, than in pretending we are not at all a part of the process.

From the above we can see that what we know about the external world, is at best (this post is already long enough without bringing up dreams and hallucinations) the same as what you experience of the external world, and your experience of the external world is incredibly dependant upon the structure of the human brain, sense organs, and how all of these interact. All of this comes back to the point that over the last 100 years the efforts of empiricism through science have shown us cogently that the views of epistemological idealism are sound and map onto how we as humans understand reality. It is impossible to talk about the external world without at least implicitly addressing the methods with which we interact with it. You cannot escape your senses and all the flaws and limits that come with them. The contents of human knowledge are inescapably determined by not only the structure of the human brain, but the structure of human sensory perception as whole, which together to a physicalist, is the structure of human consciousness.