There cannot be an infinite succession of causes and effects without any ultimate cause, because such a series would lack any cause or reason for the whole causal chain.
There cannot be an infinite succession of causes and effects without any ultimate cause at all, the argument runs, because this would fail to provide any cause or reason for the whole series or causal chain. That is to say, we need to explain “why this particular succession of causes existed from eternity, and not any other succession, or no succession at all.” Clearly the series cannot be produced by nothing. We may conclude, therefore, that the universe must have arisen from some “necessarily