A Problem of Perspective
Einstein used a thought experiment to fundamentally change how scientists thought of time. By imagining what observers would observe from different perspectives, he was able to deduce the theory of general relativity, and prove that time was not constant, but experienced differently by different observers.
The same approach has been used to try to understand the nature of human consciousness. Many thought experiments have been proposed, providing hypothetical “what if” scenarios. But where Einstein was able to use his thought experiments to eventually lead to a coherent, provable and useful framework, such scientific understanding of consciousness has yet to be achieved.
Why are Certain Lifeforms Sentient?
Imagine a planet with only single-celled life. These single cells would be “alive” in the biological sense, but they would not be sentient – in the same way that skin cells are alive but not sentient. Over time, mutations would occur among these cells, and the cells that most efficiently reproduced would become dominant in the population, and the next generation would therefore come mainly from these best reproduces, the biologically “fittest” cells. This single-celled life would diversify to exploit different environments and niches, and certain single-celled organisms would become double-celled then multicellular organisms.
This process could all occur without any of the lifeforms attaining consciousness and being sentient. So why would any organism ever become sentient?
The theory of natural selection provides the most general answer to this question. Organisms become sentient because natural selection must select for it in some circumstances. Fundamentally, the reason why humans are sentient is the same as why elephants have trunks, because being sentient must provide an evolutionary advantage.
So what is the evolutionary advantage?
The theory of evolution explains that, if an animal has a trait such as a long neck, natural selection must have chosen for it. In the case of a long neck, researchers can observe the animal, and come up with a theory as to the necks’ “purpose”, such as, allowing the animal to eat leaves located high up. Not every trait’s purpose is immediately clear, like why a peacock would have massive tail features despite being prey to numerous other animals. But that doesn’t mean there isn’t a purpose, just that it has yet to be discovered.
How does being sentient benefit an organism?
While natural selection provides the general reason that living organisms are sentient (i.e.: it conveys a survival advantage), it does not provide the mechanism behind that advantage. Why would a non-poisonous moth be brightly colored? Does the coloration help it attract a mate, or does it resemble another moth that is poisonous and thereby fool predators? Either or both possibilities could be true, and only a mix of theorizing and observation can reveal the truth.
Single-celled organisms can move toward food and away from harm; they can even engulf prey. A nematode worm, Caenorhabditis elegant  is controlled by 302 neurons, all of which have been mapped. The little worm operates like a little automaton, made from biological parts. Even in organisms that are sentient, numerous forms of “decision making” occur unconsciously, so why are all decisions not made subconsciously? Why do humans have a consciousness that experiences and perceives the world, when a nematode can just connect together its 302 neurons?
The first step to answering this question might be to look at humans and see what tasks they consciously do. Humans control their hands and feet, but not their livers or kidneys. People can’t actively choose to forget information, even information that they may know is useless.
Humans are sentient because it allows them to perform the tasks they perform consciously better than they could be performed unconsciously.
If that is the why humans (and other sentient lifeforms) are sentient, how are they sentient?
Robert M. Sapolsky wrote a truly massive, interdisciplinary book on human behavior. In it, he attacks the idea of what he calls the homunculus, the idea that “in a concrete bunker tucked away in the brain, sits a little man (or woman, or agendered individual), a homunculus at a control panel” (pg 588).
His main rationale for there not being a “homunculus” seems to be that, since he can explain the brain at every level of function, there isn’t a homunculus because there doesn’t need to be a homunculus. But this explanation leaves unexplained the very observable fact that we are ourselves, indeed, sentient. If everything is as determined as a nematode’s brain, then life wouldn’t need to be perceived at all, it would all run as unconsciously as a stomach digesting food or kidneys regulating insulin.
The crux of Dr. Sapolsky’s mistake seems to be found in the footer of page 583, where he states: “And one thing I’m not going anywhere near is this New Age-y notion: “Of course we have free will. You can’t say that our behaviors are determined by a mechanistic universe, because the universe is indeterministic, because of quantum mechanics.” Argh. What anyone sensible who has thought about this will point out is that (a) the consequences of the subatomic indeterminacy of quantum mechanics (about which I understand zero) don’t ripple upward enough to influence behavior, and (b) if they did, the result wouldn’t be the freedom to will your behavior. It would e the utter randomization of behavior”.”
But the bigger problem with his argument is that it assumes that the effects of all quantum indeterminacy are totally random. By so doing, he also assumes that the brain doesn’t use quantum effects to function. But it may. Think of how a sound can startle a person, not because a great amount of vibration occurs but because the brain is designed to amplify its effect. In fact, quantum effects may be how “free will” is manifest in living beings. It is likely the only way free will could exist, as most of the brain’s neurons are connected in neural networks with fixed outputs for given inputs. But free will does exist, and is plainly observable. The only reason anyone would need to deny it and argue it was “merely an illusion” is if they wrongly assumed that because the majority of the brain has fixed inputs and outputs, all of the brain works in this manner. (This video explains why the scientific method generally shouldn’t be used to understand things like free will: https://youtu.be/3uJkd54p9dY)
At one time, this is how brains worked. In a nematode, it is likely still how brains work. But current understanding of the quantum world is far too limited to rule out the idea that quantum effects could meaningfully affect the brain.
Why all this matters.
There are billions of humans alive on this planet. Of all these billions of humans, you experience the world from the perspective of only one. That basic question, of why you are who you are and not someone else, is one science can’t answer.