Determinism is the position that all events in the universe come from prior causes, those causes themselves are prior causes of other events, and so forth. This all goes back until the beginning of the universe (or whatever the beginning is, even if there is a beginning). In this way the universe is like a giant celestial clock. It is purely mechanical, one event leads to another, and that is all what happens in the universe. Thus, when it rains tomorrow, this was all predetermined billions of years ago.
Famous determinists include the physicist Albert Einstein, the philosopher Baruch Spinoza, and the mathematician Pierre Laplace. Determinism was popularized by Spinoza who later influenced other people to treat this idea more seriously.
But there are people who do not accept this idea. The determinists are a minority. Indeed, to be a determinist it is necessary for one to reject theism. Theism holds that God gave people the ability to shape their future, if they are obedient they will have a good future, if they are rebellious their future would be unpleasant. Determinists reject this notion because people cannot shape their future, for their future has been predetermined from the very beginning (assuming there even was a beginning). Since a very large majority of the world consists of theists, determinists are a minority. But even among atheists there are some who still reject this idea. I do not know how many atheists take the determinist position, but from my experience it is very common to come across an atheist who consider determinism to be wrong. We will ignore the theistic objections to determinism because theism is silly, and the objections we will consider will come from a secular point-of-view.
I want to give arguments in favor of determinism because I too am a determinist. There is really just one argument for determinism. Things do not just magically happen for no reason. There were laws that were behind some event taking place, the event did not just jump into existence magically. It is rather silly to think that something like this can actually happen. Determinism is the default position to take.
So what are the reasons then that people give in objection to determinism? I will focus on two arguments which are generally used. The first one is that people say that they can choose to act how they want to act. They can decide if they raise their right hand or their left hand. They can just jump around like crazy, surly this cannot be determined if they can choose their behavior, can it? The second objection that people give is that the quantum world operates through random process which we cannot determine, in fact something like the uncertainty principle prevents us from being able to determine the outcomes on the quantum level, and so the universe is not deterministic.
Let us handle these two common objections. The first objection about people choosing to act how they want to act is an easy one to answer. Consider this question. Where do your thoughts to act the way you do come from? Do your thoughts magically and spontaneously jump into existence? No. Your thoughts are determined by your brain. And your brain is determined by mechanical processes that take place in it. Thus, your thoughts themselves are the result of purely mechanical processes that emerge into your thought patterns. Your thoughts are not uncaused causes. In fact, modern neuroscience points to exactly this conclusion. When people pick a choice, say between numbers 1 and 2, an fMRI is able to show within 6 seconds exactly what choice they will take. Thus, their decision is not spontaneous, but rather a result of some mechanical process in the brain that can even be studied scientifically using modern techniques.
The second objection is a lot deeper. I have some thoughts on this objection that I did not really hear other people say. So I will introduce a new word to make clearer what I am trying to say. Let us define "determinable" to mean "a deterministic process that can be computed".
Notice the difference between "determinable" and "deterministic". Determinability is a stronger condition than determinism. Determistic is simply the statement that all events have prior causes in an unbroken chain of cause and events. Determinability is the statement that these events can be computed. There is a difference between Laplacian determinism and Spinozian determism. For Spinoza determinism meant exactly what I meant by "determistic". For Laplace determinism meant a lot more. For him it means that we can compute the determined outcomes. Laplace made this statement clear with his thought experiment of the "Laplacian demon". A creature that knows all the particles and all the information of the universe. Laplace said that such a creature, if given the information and the computing power, would be able to compute all the determined events. Laplace was a mathematician, Spinoza was just a philosopher. It is not a surprise way Laplace had a much stronger form of determinism than Spinoza, because Laplace himself was capable of computing certain deterministic events.
Determinability is not possible, not to its full extent. Laplace did not know this, he lived in the 1700's, he did not know quantum mechanics nor did he know anything about Chaos. In the 1880's one of the great mathematicians, Jules-Henri Poincare' discovered the instability of the three-body problem. The three-body problem was solved (by one of my favorite mathematicians) Leonard Euler. But Poincare' managed to prove that the solutions are not stable, which meant that from a practical point of view it is not possible to compute the outcome of a three-body problem. It is still however deterministic, if we perfectly knew the measurements then we would be able to determine the outcome, but because our measurements are approximations and approximations are not good enough we can never really solve the three-body problem. This is a classical physics problem that does not fall into the category of determinable, but it is nonetheless still deterministic.
The quantum world presents different difficulties which lead to non-determinable events. But just like with the three-body problem it does not mean that these events do not come themselves from prior causes. It just means that we have limitations to what we are able to compute. Thus, what I am trying to say is that the universe is deterministic but not fully determinable.
There is a thought experiment that we can consider that will prove that determinability to its full extent is impossible. Let us assume that determinability is always possible (given enough computing power). If determinability is possible then we can write a book about all the major actions Laplace will ever take in his life. And we will indeed see that Laplace will follow the exact outline of the book. But this determinable result is assuming that Laplace does not know the information which is in the book. Suppose that we planned to determine all the major events Laplace would ever take in his life and we gave him that book, i.e. one of the major events is him reading the book on his major choices in life. Upon reading this book Laplace will violate the outcomes in this book. If the book said that we will buy a house in France he will rebel and purposefully buy a house in Germany to show that the book is wrong. No matter what we write in the book Laplace will be able to read and disobey. So we have something very interesting. If Laplace has no knowledge of our determinable predictions then he will act perfectly in accordance with these predictions. But if Laplace knows about our predictions then he will not act perfectly in accordance with our predictions, he will violate them. It is impossible to give Laplace a book of his major life choices and expect him to follow the book. Therefore, the determinism problem of determining how a person would act if given the information about his actions is impossible. There is no way to compute this. And it is in this way that determinability to its full extent is impossible. This does not mean however that determinism is invalid, it just means that determinability is not always possible.
The last thing I want to talk about is randomness. People object to determinism and say that the universe is just a bunch of random particles colliding with one another, randomness is not determined, so determism is false. These people who use this objection do not understand the meaning of "random".
What does the word "random" even mean? It really depends who you ask. The physicist might answer differently from the mathematician. But I think the mathematician is more accurate in this instance. The physicist will answer that "random" means events which are undetermined, I have seen textbooks that actually describe randomness by this incorrect description (for example, the terrible book on Mathematical Physics by Arfken and Weber). The mathematician's definition is much more precise and useful. "Random", as it means in probability theory, is an event that has an equal probability of happening as all other events. For example, consider a die. The probability of throwing a six is 1/6, which is the same probability as for any other event. That is why we say it is "random". Throwing a die is not an undetermined process. If we can calculate how much force you put into the die, the height of your arm off the table, the angle by which you throw the die, and so forth ... we might (assuming it is not a chaotic problem) be able to determine the actual outcome of the die. The die does not drop uncaused, it is all determined, but it is random nonetheless.
Yes, I agree, the universe is random. However, random does not imply undetermined. It just means that particles simply have equal probabilities of acting one way as another. So in conclusion, the way I describe the universe and solve the problem of determinism is by saying that the universe is a deterministic random but non-determinable universe. I hope this clears up any confusion people have about determinism, if not, well, you can go and kill yourselves because you are rather useless people.