A fairly common way to thinking about probability is to enumerate all the outcomes and assign them each an equal probability.
IN CONTEXT
When you roll a die, there are six results since they are numbered: 1, 2, 3, 4, 5, or 6. Those are the outcomes.
Suppose you wanted to determine the probability of rolling a 2. Only one of those faces shows a 2, and there are six total faces, so the probability of you rolling a 2 is, therefore, one out of six.
The understanding here is that the faces--1, 2, 3, 4, 5, and 6--are equally likely. This is the key! There are other events whose probabilities can be calculated using this method.
Other probabilities calculated using this method are based on the key idea that outcomes are equally likely. Anytime you have outcomes that are equally likely, the theoretical probability method can be used, such as:
The basic formula for theoretical probability is:
The theoretical probability model does not work if the outcomes aren't equally likely.
EXAMPLE
You could use this reasoning to say, for example, you could get home safely from work in your car, or you could get in a horrible car crash. Either is a possible outcome. If you assign those equal likelihoods, then the probability of you getting into a car crash is one half, because there are only two outcomes. You either make it home safe, or you get in a crash.Source: Adapted from Sophia tutorial by Jonathan Osters.