{ 1 } - gives no evidence at all that the world had a beginning in time.
{ 2 } - gives some evidence that the world had a beginning in time.
{ 3 } - proves conclusively that the world had a beginning in time.

He admitted this, but he didn't think that the evidence was conclusive. So he had some doubt that the world had a beginning.

Mackie thought he had a conclusive argument for rejecting belief in God. (We'll see his argument, based on the problem of evil, later.) So he thought we should accept one of these two alternatives:

1. The world had no beginning in time.
2. The world just popped into existence, with no cause.