In the fourth and final book of the Essay, Locke sets forth the major elements included in the theory of knowledge that he has sought to establish by the arguments presented in the first three books. Many of his conclusions can be anticipated by anyone who has followed his line of reasoning up to this point. There are, however, a number of questions which arise when one stops to consider the implications that are involved in many of the statements which he has made. Several of these questions are discussed at some length in this part of the Essay.
Probability has something to do with a chance. It is the study of things that might happen or might not. We use it most of the time, usually without thinking of it. Everything from the weather forecasting to our chance of dying in an accident is a probability. Probability is a mathematical term for the likelihood that something will occur. It is the ability to understand and estimate the likelihood of any different combination of outcomes. Before planning for an outing or a picnic, we always check the weather forecast.