Consider a box with three balls: one Red, one White, and one Blue. Using an equally likely 4.3.11 assumption, the probability of randomly pulling out a Red ball should be 1/3. That is P(Red) = 1/3.

However, suppose that for a first trial you pull out the White ball and set it aside. Attempting to pull out another ball leaves you with only two options and so the probability of randomly pulling out a Red ball is 1/2. Notice that the probability changed for the second trial dependent on the outcome of the first trial.