Free will is being able to choose between different actions. If we judge an action (for example, as good or bad) it only makes sense if the action is freely chosen.

Things like advice, persuasion, and prohibition, are pointless unless people have some kind of free will. Free will means people can do different things. Different results come from different courses of action. Traditionally, only actions that are freely willed deserve credit or blame. If there is no free will, there is no sense or justice in rewarding or punishing anybody for any action.

Free will is a major problem in ethical philosophy, and is also relevant to the philosophy of science.

In ordinary life, and in law, people are generally assumed to have free will, and are responsible for what they do.

In philosophy

Determinism 
Determinism comes from the idea that our world – and the universe – is in some way rather like a machine. This idea goes back a long way in history (at least 2,500 years).

Hard determinism 
There is more than one kind of determinism, but essentially it is the idea
that events in the past fully decide (cause) events in the future. It is the same as saying "the universe is like a clockwork instrument". If you knew everything about it, you could predict exactly what will happen. To illustrate this Pierre-Simon Laplace proposed a thought experiment in 1814, which he called Laplace's demon.  If determinism is the case, then there can be no free will.

The view that a deterministic universe means people do not have free will is called "incompatibilism". It means if determinism is true, it is incompatible with free will, and so free will does not exist.

Soft determinism 
Many thinkers do not like what follows from hard determinism, and ideas have been put forwards as to why we do have free will. Here we give just one of these ideas.

Soft determinism (or "compatibilism") tries keep determinism, but still claims that free will is possible. David Hume had this position. According to Hume, free will is not the ability to make a different decision under the same circumstances. Because there may be slight differences in the circumstances, a different decision can be reached. Chrysippos. a stoic philosopher gives the example of a dog which is tied to a cart. This dog can freely decide to follow the cart. William James coined the term "soft determinism" in  The dilemma of determinism in 1884. There, James writes "A common opinion prevails that the juice has ages ago been pressed out of the free-will controversy". James went on to argue, just as did Plutarch, that events fall into two groups: the causally determined and the rest.

In law
The law assumes we have free will. The job of courts is to find out when people do things and what they were thinking when they decided to do it. For example, think of someone who kills someone else. A court tries to figure out (1) if he or she actually killed the other person, and (2) if he or she decided to do it. The courts do not ask the philosophical question above.

In science

Physics
In the past, people such as Democritus saw the universe as deterministic. Some people thought that getting enough information would allow them to predict perfectly what will happen in the future. Modern science, however, is a mixture of deterministic and stochastic theories.

References

Philosophy
Ethics
Causality