Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we are
not certain. The proposition of interest is usually of the form "Will a specific event
Event (probability theory)
In probability theory, an event is a set of outcomes to which a probability is assigned. Typically, when the sample space is finite, any subset of the sample space is an event...

 occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The certainty we adopt can be described in terms of a numerical measure and this number, between 0 and 1, we call probability.