Why the Universe Must Have Had a Beginning

Philosopher William Lane Craig shares the paradox of the Grand Hotel (a veridical paradox developed by mathematician David Hilbert in the 1920s):

Craig uses illustrations like this to show that actual infinities are impossible, supporting the second premise of his version of the Kalam Cosmological Argument:

Everything that begins to exist has a cause.
The universe began to exist.
Therefore, the universe has a cause.