Think of the real number line. We are at some point t that represents our current time. If time goes back forever, that is had no beginning then it means (-∞,

Think of the real number line. We are at some point t that represents our current time. If time goes back forever, that is had no beginning then it means (-∞, t] represents all the time up to t. How did we get to t? I would take an eternity to get to t-1. Indeed, for each moment in time it took an eternity to get there. Hence time must have had a beginning.