Living things are natural machines that manage to replace themselves by growing copies of themselves which will continue to live once they themselves have died. Most of them (particularly plants) are just massively complex chemistry sets, and even most cells in animals are nothing more than a bunch of chemicals and chemical reactions. The only place where anything special happens is inside the brains of animals (including birds, fish, etc.), and that special thing is called consciousness. Consciousness is the part of a living thing that experiences feelings - it's the part of you that is the real you. You can lose your arms and legs in an accident and yet you are not lost along with them because you do not live in your arms or legs. People used to believe that the soul (the conscious part of them) was located in the heart, but people who have heart transplants don't suddenly become someone else just because their heart has been replaced with someone else's. These days we know that the soul (meaning the thing that is conscious) lives somewhere in the brain.
Consciousness is one of the biggest mysteries left in science, and most of the experts in this field of study talk absolute nonsense about it, though that's hardly surprising given that it's so hard even to begin to imagine what consciousness might be. We know that parts of the brain can die without consciousness going away, but there doesn't appear to be any central part anywhere in the brain which we can point at and say that consciousness resides there. There is no special atom or particle lurking at the centre which calls the shots. The soul does not seem to be a single item, and yet it feels as if it ought to be a single piece of something: surely your soul can't be split in two?
My own semi-theory of consciousness is that every piece of matter is conscious, though most of them are sitting around in non-living things like rocks, planets and stars, so they might only experience random feelings and have no sense of any passing time. Even the particles of plants would only experience random feelings, and they are not related to anything that is happening to the plant in any useful way. Material in the brains of animals is very different, however, because brains are designed to load the consciousness of the material of which they are made with data which means something real to the animal. The conscious feelings of the matter in the brain are no longer random, but can be made to feel pleasant, unpleasant or neutral sensations, and these sensations can come in many different flavours and intensities. Different flavours of feelings are mapped to different kinds of events, so the neutral feelings that we call colours have been mapped to neutral information that comes from our eyes, while the unpleasant feelings that we call pain have been mapped to damage-detection sensors. Clearly an animal would not survive for long if pain was mapped to the sensors in the mouth that detect tasty food, or if pleasant sensations were linked to the damage-detection sensors: the mapping of different kinds of sensations to different kinds of information has obviously had to evolve in such a way as to aid the survival of the animal.
My theory suggests that there is more than one consciousness in a brain: indeed, there must be millions, billions, trillions, quadrillions or quintillions of them, and all of them mistakenly feel that they are the only consciousness in that brain. I suspect that the brain uses these consciousnesses as an averaging mechanism to make decisions: different bits of the brain load the material of the brain with feelings which spread out through the rest of the brain, and then the brain reads back the average feelings at some point far from the inputs in order to get the answer as to how it feels overall. None of the conscious bits of material will ever know if the decision went for them or against them, because they have no mechanism for remembering what they wanted at the time the decision was made, and there will be new inputs based on the result of the decision which will flow through the material of the brain and make all of it feel happy with the result of the decision and that the decision must have gone the way it wanted.
That may account for what goes on in the consciousness of a simple animal, but there are more complex features of consciousness in the higher animals which are harder to account for. We seem to be able to understand complex ideas in a fully conscious way. To some extent this can be shown to be an illusion, but it's hard to pin down just how much can be explained away in such a manner. I am still working out experiments to test the limits of my own consciousness, and I will let you know what happens if I get any nearer to an answer. In the meantime, you might like to try some experiments of your own: you could start by trying to work out how many things in front of your eyes you can actually be aware at the same time. Is it a great many, or might it be as few as two? I actually wonder if it might only be one, but it's hard to tell because we seem to be able to switch our attention between different things so quickly that it gives us the illusion that we are conscious of many things at once, but I'll leave you to make up your own mind about that. You might not realise that almost everything you can see is blurred and that it's only in the centre of the central area of what you can see that things are fully sharp, and yet you generally feel that everything is sharp at once. I am sure of one thing, though: an intelligent computer would not need consciousness to work. Our feelings are used as a rough-and-ready calculation mechanism, but we often have to go against our feelings when reason tells us that our feelings are wrong. Feelings are a primitive mechanism, whereas our intelligence is a more recent innovation which tends to come up with better answers, though decisions based on feelings are often quicker and easier to make, and they may save your life. Computers are fast enough that they can calculate everything properly without needing to fall back on an ancient system originally designed for controlling the behaviour of simple animals, but there is nothing in the machine to call itself "I", so it cannot be thought of as being alive in the way that we are.
If I am right about the way consciousness works, then it may mean we are not who we think we are. All the atoms in your brain are replaced from time to time by new ones which come into you in the food that you eat, so the parts of you that are consciously aware of being you right now may not be the same ones as were there a year or two ago, and indeed it is likely that the child that I remember being long ago, I never really was at all: I have his memories written into the structure of my brain, and so when I remember things from the past I re-live them in my mind and feel that they really are my own memories, but it's a mere illusion because I wasn't there at the time when the memories were made. The implications of this (if it's true) are extraordinary: if someone killed you, some of your conscious particles could end up finding their way into the head of the person who killed you, and so you could end up serving time in prison for a crime of which you were the victim! Think about this too: many of the pieces of conscious matter in your head have been inside the heads of many people before, all the way back through history. Every single one of you has pieces of matter in your head which now conciously believe themselves to be you, but in the past they used to be conscious of being Adolf Hitler, Joseph Stalin, and indeed every other mass-murdering dictator who's ever lived, including all the ones who are still alive. You also have conscious material in your head that used to be in the heads of everyone else who has ever lived, so long as they have been alive long enough for enough material in their heads to be replaced and for it to have come to you in your food. You have conscious material in your head that has come from every other kind of living thing too, so there are parts of you, which now believe themselves to be you, which used to be conscious of being dinosaurs, and indeed you will have some from practiacally every individual dinosaur that has ever lived. This may sound unlikely to you for a number of reasons, but the logic behind it is robust. Atoms are so small that if you throw a cupful of water into the sea and wait a few years for those water atoms to spread out through the world's oceans, you can then scoop a cupful of water out of the sea anywhere in the world and it will contain a number of atoms which were in that original cupful of water that you threw into the sea all those years before: this is possible because there are more atoms in a cupful of water than the number of cups required to hold all the water of the sea. The same applies to the number of attoms in your head and the size of your head relative to all the amount of material in circulation that can be used to make living things: it's all recycled to make new living things, and it doesn't even wait for an animal to die before it moves on again. We may be much shorter-lived than we normally suppose, and yet at the same time we are much longer-lived as well because we have been living here for hundreds of millions of years in a succession of different forms.
There is a serious problem with my consciousness theory: there is no mechanism by which conscious material could tell the brain that it is conscious, so it should be impossible for us to know in any intelligent way that we are conscious, and none of us should therefore be able to talk about it. This is one of the biggest puzzles in the world, and whoever solves it will become one of the most famous people of all time. It may be that there is no such thing as consciousness and that the whole idea of consciousness is a fiction made up by the brain, but it's hard to see why such a fiction would be an advantage, and it's also hard to imagine that we are being fooled into thinking we feel pain. If it turns out that it is just a fiction, then there can be no such thing as suffering, and if there is no such thing as suffering, then there is no need of morality either (no such thing as good and bad) because it must be completely impossible to harm anyone.
We've strayed a long way into philosophy rather than science, so let's get back into some hard facts. All living things on the Earth use a chemical called DNA as a kind of construction manual for making new plants or animals of the same type as themselves: DNA takes the form of extremely long molecules with four different kinds of chemical links in them which are used to encode data. The data can be read by special chemicals which behave in different ways according to the data they are reading at the time, and different parts of the body can be set to read different parts of the DNA so that they can perform different tasks, so brain cells will read parts of the DNA which relate to setting up thinking equipment, while muscle cells will read parts which relate to enabling movement. Scientists are still trying to work out the details of how the system works, but it is essentially a natural computer which appears to have designed itself through a long series of chance events (though it's a computer with a very specialised and limited functionality). The DNA is the program code that the computer runs (though more as a recipe for construction rather than to control thinking tasks), while chemicals in cells are the machine that rus the program. Some scientists think it is such an amazing mechanism that it must have been designed by God, but other scientists are carrying out experiments which are making it look increasingly likely that it could have come about through a series of chance events (evolution by natural selection). The most serious question left about the system is how the first living thing came into being (because more complex things could easily have evolved from a simple one), but major breakthroughs are now beginning to be made in this field. I suspect that the simplest possible living thing would be much more simple than many people imagine, because it would be in a simpler world in which there is nothing around waiting to eat it: it might not even have needed to be protected by a cell wall, so that in itself would enable it to be far simpler than any living thing that exists today.