Probability is a numerical description of how likely an event is to occur or how likely it is that a proposition is true.
Probability is the mathematics of uncertainty, proportions are the mathematics of probability.
Probability - Initial Definitions
Probability - Views on Probabilities
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4 Views on Probabilities Complement Each Other
If you know that a physical process produces certain outcomes with certain long-term frequencies, it’s natural to think that the process has the physical propensity to produce these outcomes with that frequency. Then it’s also natural to adjust your degrees of belief (expectations) for each outcome, so they match that propensity. Or you can go the other way around and form your expectations for the frequency of occurrence of an outcome (by appealing to the law of large numbers) after a logical analysis of the sample space.