There is an old saying in philosophy, which is that metaphysics is not a problem to be solved, but a disease to be cured.

Metaphysics is notoriously tricky, but eminently solvable.

The definition of metaphysics is the study of the nature of reality. This naturally begs the question: what is reality?

Now there are definitely challenging problems in philosophy, such as the definition of ethics and virtue, and morally appropriate actions in ambiguous circumstances, and so on. However, philosophy should at least be able to answer the question that is easily and instinctually answered by every other living being. Single celled organisms can tell the difference between themselves and others, between food and debris – insects can navigate from one continent to another – as can birds – and babies can very quickly figure out the difference between themselves and the world around them.

It is reasonable to hope that philosophy can answer a question easily solved by tiny-brained organisms. Ethics is a discipline peculiar to human beings, but the need to differentiate between dreams and reality is common to countless creatures who never need to bother their minds about virtue.

In other words, if little brains can solve a problem effortlessly, surely big brains can solve it as well! If a six-month-old baby can lift a weight, surely an adult can as well.

The failure of philosophy to resoundingly answer a question easily solved by your average bee has not done much to enhance the reputation of philosophy. The general perception that philosophy is an abstract and useless discipline that has little or nothing to do with practical life is only further embedded in our minds when we wake from a nightmare, knowing for certain it was not real, and philosophers wag their fingers at us and claim that we have no way of actually knowing that!

The vast majority of people have no trouble whatsoever navigating empirical reality – in fact, those who claim they cannot do this – who see visions or believe that they can fly – are defined as mentally ill, and treated as such.

The everyday tools and machines that we use to navigate empirical reality and survive within it are all designed, created, marketed, transported and sold by people with a full and complete ability to differentiate waking reality from subjective impressions. Try giving a child the "idea" of candy rather than a actual piece of candy, and see how he or she reacts. Even a toddler has no trouble differentiating between the contents of a mind – an idea – and something in reality – a piece of candy.

We can only hope that giant-brain philosophers can achieve the empirical wisdom of an 18-month-old toddler.

The first thing to understand about metaphysics is that it is the study of everything in the universe except the human mind. Subjectivity by definition is that which occurs within the mind, and therefore is not reflected or reproduced in external reality. You may prefer the colour red, I prefer the colour blue – the objective wavelengths representing red and blue exist in the world, and can be measured objectively – but the preferences exists only in our minds.

Now you might be screaming at me and claiming that I am begging the question or jumping the gun, because I have just casually tossed off a definition of subjective and objective without proving anything – and you might cry that these are all just assumptions without evidence.

If you've ever played golf, and really worked to improve your swing, one of the things a coach will do is take a video of you, and then slow it down so that you can see every detail of your movements. Slowing down to see the details is essential for improvement – in both sports and thought.

You are reading this using your eyes, or listening to it using your ears. Your reaction that I have not proven anything proves that you have already proven everything.

If you say that I have not proven anything as yet, then when we slow down your thought process, we can understand that you have fully and completely accepted the following:
– I exist
– I am responsible for what I have written
– language has meaning
– evidence is required for proof
– the senses operate in an objective manner – in this case, your eyes reading my words, or your ears hearing them
– an objective reality exists that both you and I inhabit

There is more – much more – but these are the most essential facts that you accept in order to both read and respond to my writing. The moment that you correct me – or object to my arguments – you have established every root premise required for the acceptance of objective and empirical reality.

If you disagree with me, and want to argue with me – rather than with yourself – then you accept that I exist independent of your mind. (You also accept property rights, by the way, because you accept that I am responsible for the products of my actions – that it is MY argument.) By demanding that I provide proof, you are demanding that I conform to an objective standard of reason and evidence, and that something is not true just because I say it is true. In other words, you accept a standard independent of consciousness to validate truth claims made by a human mind. (Now, it could be the case that you believe that another personality within your own mind wrote the arguments above, in which case you will never contact me to correct me, or never tell anyone that I am wrong, but rather will descend into arguing with yourself, which will remain invisible to everyone else.)

If both you and I exist, and language has meaning, and arguments must conform to objective standards in order to be valid, and an objective empirical reality exists that we both inhabit, and the senses are valid – what answers required by metaphysics have not been provided?

The question of whether other minds exist is easily answered – does knowledge exist in the world that you have not yet attained? I don't speak Japanese, but I accept that the Japanese language exists, and is valid, and is spoken by millions and millions of people. I do not have the mathematical ability to plot a course for a spaceship to pass by the planet Pluto, but I accept that others do have this ability, because spaceships have passed by the planet Pluto. I do not have the ability to forecast the weather, but I accept that weather forecasters exist, and are usually accurate. I do not have the ability to build a computer from scratch, and program voice recognition software to type out what I am saying, but this is how I am producing this article, so I accept that other people exist, who possess knowledge that I do not. Songs come into my ears that I did not compose, I consume books I did not write, and I eat food that I do not know how to produce. I drive a car I did not create myself, cooled by an air-conditioning system I do not understand, powered by gasoline that I did not dig out of the earth.

Since I am perpetually surrounded by the products of minds not my own – and in fact I need to accept the products of minds not my own in order to make this argument and share it – it would be truly perverse for me to deny that which I require to make my argument. This would be like a jazz trumpeter claiming that he did not need to breathe in order to produce music, or a dairy farmer claiming that he did not need cows to produce milk.

So - if minds exist other than my own, the question must be: where do they exist? Remember, I said that metaphysics was the study of everything in the universe except the human mind – so naturally, if minds other than my own exist, they must exist in an empirical, objective realm.

Why must they exist in an empirical realm? Well, simply because I have no way of confirming the existence of other minds without using the empirical evidence of my senses. Since telepathy does not exist, I must read the products of other minds through their writing, or view their paintings, or listen to their music, or touch their fabric, or taste their food. Every single piece of evidence for the existence of other minds comes to me through my senses. Now, if I reject the evidence of my senses, then no one has to worry about my arguments for very long, because I will walk into traffic or step off a building or refuse to eat or drink. If you write to me and tell me that I am wrong about something, I will simply tell you that I never said such a thing – if you provide me evidence that I did, I will simply say that your senses have deceived you into misinterpreting that evidence, and we will never have to discuss anything of any philosophical or intellectual import. By rejecting the evidence of the senses, you exclude yourself from any participation in the realm of philosophy, since all philosophical material must be communicated through the evidence of the senses. Making the philosophical argument that you reject the senses is as truly insane as taking a train journey with me in order to prove that trains do not exist.

And even if you pull a full Cartesian argument, and say that your senses are all being manipulated by an external being of some kind, that you are really a brain in a tank being controlled for some unknowable purpose – you still have not proven that the senses are invalid, because in order to be controlled, that external being must have used the evidence of its senses in order to accept that you exist, and to wire you up to manipulate you, and to make sure your brain receives enough oxygen and nutrition to stay alive – and all of this must be accomplished by the acceptance of objective reality provided through the evidence of the senses for the controlling Demon hellbent on manipulating you.

In the same way, a blind man cannot both demand a seeing eye dog, and claim that there is no such thing as vision in any animal. If his own eyes do not work, but he wants a seeing eye dog, it's because he accepts that the dog can in fact see. His deficiency does not prove that eyes do not work, but only that his eyes do not work. In the same way, if you are a brain in a tank being manipulated, and therefore cannot trust the evidence of your senses, this does not prove that the senses are invalid, since the being that is controlling you can only do so by accepting the evidence of its own senses.

A proposition can never be accepted as true if there is no possible disproof. If we combine this principle with the principle of Occam's razor, we can easily throw aside the Cartesian argument that our supposedly objective reality might just be a complex manipulation inflicted on us by an external being. In other words, if we have a perfectly rational explanation that only requires one reality, claiming without any standard of disproof that there is more than one reality is invalid. If we claim that we are a brain in a tank being manipulated by an external Demon, and any standard provided that could disprove this hypothesis is rejected out of hand, then the claim fails, because that which is asserted without evidence can be rejected without evidence. Also, as I wrote in my previous book Essential Philosophy, the proposition that everything we experience could be artificially created by an external Demon fails due to Occam's razor, because it falls under the umbrella of "infinite regression." If we are being manipulated by a Demon, there is no rational reason why that Demon itself is not being manipulated by an external Demon, and so one and so on to infinity. In other words, we would need infinite universes, which means infinite energy, infinite space and infinite time, in order to even remotely be able to sustain our hypothesis – when none of this is necessary, because we only need one universe for the "objective reality delivered through the evidence of the senses" hypothesis.

Of course, if someone genuinely believes that he is a brain in a tank being manipulated by an external Demon, he would never bother to share this hypothesis, because he would only be making the argument within the experiment of the Demon, which the Demon already knows is an experiment, and accepts the facts. I don't bother making the argument to a fellow airline passenger that airplanes exist, and that flight is possible, because we are already in the air, so he naturally accepts that as a reality. In the same way, if you are just a phantasm created by a Demon in my mind, I'm not going to bother trying to convince you of that, because the Demon which controls you already knows that that is true. That would be like trying to convince a puppetmaster that he controls his puppets – he already accepts that he does, so you're not convincing him of anything he does not already know.

It is certainly possible to reject every shred of evidence against the "Cartesian Demon" hypothesis, and maintain that it is true – or possible – no matter what, but then we face the empirical challenge of someone both claiming that you do not exist, while at the same time wanting to change your mind. This can only be shrugged off and walked away from – a person who cannot recognize this essential contradiction is beyond reason, and therefore beyond hope.

Furthermore, for those of us who accept the prosaic reality that our minds require sustenance, and that sustenance is only available in the objective world, it follows exactly that anyone and everyone who argues against the validity of objective reality has spent their whole lives finding sustenance within that objective reality, in order to be alive to make the argument against it. It is even crazier than slapping me in order to prove that human beings do not have arms. If a 40-year-old man is arguing with me, claiming that objective reality does not exist, I know with absolute certainty that he has spent 40 years gaining sustenance from objective reality, since we cannot fill our bellies by imagining food. Since he is only alive because he accepts objective reality, arguing against objective reality is arguing against his own existence. If someone screams in my face that he does not exist, and sound does not exist, and I do not exist, then I take some slow and very careful steps backward, because that person has lost his mind, and is extremely dangerous.

If someone uses the evidence of my senses to attempt to disprove the validity of my senses – in other words, they make an argument – then they are equally deranged.

If someone truly believes that I am an imaginary avatar created by a manipulative Demon – and then attempts to convince me of that fact, then they accept that I exist, and do not know that I am the product of demonic manipulation. However, if I do not exist, then they are merely talking to the Demon, which already knows that it is manipulating that individual. This is about as sane as attempting to convince a prison guard that you are in prison, and he is guarding you. We can only pity the derangement that would produce such twisted and distorted arguments.