Below we will consider a few lines of logical thought. They will all take the form of proof by contradiction. This type of proof assumes the opposite of what it wants to prove, and then shows that this assumption leads to a contradiction. A contradiction occurs when a proposition is said to be both true and false at the same time and in the same sense. It is fundamental and obvious that contradictions cannot be true. For example, the material world cannot be the only world that exists and also not the only world that exists. If some proposition necessarily leads to a contradiction, then that proposition must be false. Thus, its opposite is true. So there’s some introductory logic for you.