Many people have tried their hand at solving the puzzle of mind and matter. What are they? Are they the same thing, looked at in two different ways? Or are they totally different from each other? If so, how do they interact? Or is their apparent separateness just an illusion?
Some people say there's no point in asking these questions because, how can any of the answers pay your rent? get you your next meal? find you a partner you can have children with? These people may be right, but the mystery has nevertheless fascinated some of the greatest thinkers.
An area of modern life which is directly touched by this branch of philosophy is artificial intelligence (AI).
When you think about it, don't you find it a bit amazing that you can just have a thought in your mind about drinking a cup of tea, and instantly things begin to happen in the world of matter outside you. Your arm begins to move, your fingers to grasp, the cup moves in an exact trajectory to reach your mouth. How can this happen? Or Beethoven just thinks about his Fifth Symphony, and that produces notes on a paper, and orchestras play, and people's ears vibrate, and the ideas of Beethoven are now in OUR minds when they weren't there before.
One of the most influential thinkers of all time was Rene Descartes. He it was who made the famous statement "I think, therefore I am." A lot of people maintain that he said it in Latin as "Cogito, ergo sum," which would have been quite cool, only he didn't. Actually, he said, "Je pense, donc je suis," because of course he was from France. The idea is that the only thing you can be absolutely certain about is that you are thinking something, so there must exist a thinker to do the thinking. After that, everything else is inferred from this first thought.
Descartes was a dualist, he believed that mind and matter both exist, but they are completely different entities, so he had to solve the problem of how they interact: how it is that you have a thought about a doughnut and pretty soon your body is moving to the doughnut store and joining up with a doughnut. Descartes decided that animals were pure automatons, like machines, but humans are different, in that they have a soul. He believed this soul was located in the pineal gland, a small organ in the brain (which is now believed to be a relic of evolution with no use whatever, like your appendix) and that this was where the interaction takes place. Apart from the fact that no-one believes the pineal gland theory anymore, this idea also has the disadvantage of introducing yet another entity, different again from the original two, making things more complicated rather than simpler.
Descartes' view, and those that are similar, are called interactionist theories. A successor of Descartes, Arnold Guelinex, proposed a theory of psychophysical parallelism. Imagine two clocks, each keeping perfect time, but you can only SEE clock A and HEAR clock B. Then every time you see clock A point to the hour, you will hear clock B strike, making it seem that one has affected the other, when in fact they are quite unrelated. So it is with mind and matter; body and mind are separately "wound up" by God at the beginning of creation, so when you get a desire to walk, at the same time, purely unrelated physical events happen in your legs to cause them to walk. Another theory is that when events happen in the material world, then God affects us so that we think we are being affected by them.
All these ideas make us out to be not much more than puppets, with God pulling the strings. Wouldn't it be easier to have created a world where mind and matter are the same thing, so they didn't need to interact? Spinoza tried to solve this by saying they didn't interact, but they were like two sides of the same coin. He actually believed that everything is God, so that mind and matter were just two different aspects of the Creator.
There are many other theories of mind and matter, but that's enough for one day.