Leonard Mlodinow: The 3 Foundations of Probability
Farnam Street stashed this in Interesting
These three principles are easy to say but hard to master!
1. The probability that two events will both occur can never be greater than the probability that each will occur individually.
2. If two possible events, A and B, are independent, then the probability that both A and B will occur is equal to the product of their individual probabilities.
3. If an event can have a number of different and distinct possible outcomes, A, B, C, and so on, then the probability that either A or B will occur is equal to the sum of the individual probabilities of A and B, and the sum of the probabilities of all the possible outcomes (A, B, C, and so on) is 1 (that is, 100 percent).