A probability must be a real number in the interval [0, 1].
The sum (or integral) of the probabilities over all possible values must be 1.
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Empirical probabilities.
Things and numbers don't have probabilities. Situations and events that can happen have probabilities.
If the events are independent then you can multiply the individual probabilities. But if they are not, you have to use conditional probabilities.
Conditional probabilities arise when you revise the probabilities previously attached to some events in order to take new information into account. The revised probabilities are 'conditional on the new information you have received'.