What Is Conditional Probability?

There are three mushrooms on a table.

One of them is poisonous.

You pick one of the mushrooms, leaving two mushrooms on the table.

The evil wizard takes one of the two remaining mushrooms, claiming that it is not poisonous (assume that he is telling the truth).

He grins evilly at you and asks whether you want to switch the one remaining mushroom with the one you had chosen earlier.

Should you switch or not?