The title of this book is Probability and Stochastic Processes. We say and hear and read the word probability and its relatives (possible, probable, probably) in many contexts. Within the realm of applied mathematics, the meaning of probability is a question that has occupied mathematicians, philosophers, scientists, and social scientists for hundreds of years.
Everyone accepts that the probability of an event is a number between 0 and 1. Some people interpret probability as a physical property (like mass or volume or temperature) that can be measured. This is tempting when we talk about the probability that a coin flip will come up heads. This probability is closely related to the nature of the coin. Fiddling around with the coin can alter the probability of heads.
Another interpretation of probability relates to the knowledge that we have about something. We might assign a low probability to the truth of the statement, It is raining now in Phoenix, Arizona, because we know that Phoenix is in the desert. However, our knowledge changes if we learn that it was raining an hour ago in Phoenix. This knowledge would cause us to assign a higher probability to the truth of the statement, It is raining now in Phoenix.
Both views are useful when we apply probability theory to practical problems. Whichever view we take, we will rely on the abstract mathematics of probability, which consists of definitions, axioms, ...