Question

From Third axiom examples

Answer 1

For any sequence of mutually exclusive events E1,E2,... (that is, events or which EiEj=∅ when i≠j ):

P( ⋃∞i=1Ei )=∑∞i=1P(Ei)

I know this is an "axiom" which is something assumed to be true. But usually there is a motivation or insight as to assuming WHY something is the case.