# Possibility and Free Will

What is possibility? What do humans mean when they say "This is possible" or "That's impossible!"?

There are many definitions of "possibility", so in polite conversation it's useful to use them precisely. Let's start with "physical possibility" as one form. This is basically saying "The known Laws of Physics either permit or do not forbid this outcome from occurring." As an example, suppose you're pondering the possibility of picking up a rock and throwing it such that it lands on the moon. This is physically impossible because your body and your arm do not contain the required energy to get the rock there. As another example, suppose you are considering the possibility of sustained nuclear fusion in the lab providing a huge supply of cheap energy. This is physically possible because we have physical proof of this process already happening--that is, the sun does it already.

Another form of possibility is "Logical possibility". This is basically saying "Using two-valued True/False logical propositions and their rules, we can logically deduce whether some proposition questioning the logical possibility of something is true (that is, it's possible) or false (that is, impossible)." As an example, suppose you assume for the sake of argument that a human arm can generate a million petajoules of energy on a whim, then you assume the fact that it takes some amount of joules to throw a rock to the moon. Then you can logically deduce that because you have a sufficient amount of energy to throw a rock to the moon, you can do so. It is logically possible under this set of assumptions. As another example, suppose for the sake of argument that penguins can fly. Then it's logically possible for a penguin in a zoo to take off and fly back to its home in wherever it came from. As another, assume for the sake of argument that crows can't fly, and so it's logically impossible for it to escape a glass container you put it in.