He said:

“If it is objected:

‘The decisive proof that an infinite series of causes is impossible is as follows: Is each individual cause contingent in itself or necessary? If it is necessary, it has no need of a cause. If it is contingent, then the whole is characterized by contingency. Everything contingent requires a cause distinct from itself; therefore, the whole requires a cause distinct from itself, and thus requires a cause external to those causes.’

We reply:

‘The terms “contingent” and “necessary” are ambiguous, unless “necessary” means that whose existence has no cause, and “contingent” means that whose existence has a cause. If this is what is meant, let us return to this wording and say: Each individual cause is contingent, meaning that it has a cause distinct from itself. But the whole is not contingent, meaning that it has no cause distinct from itself and external to it. If the term “contingent” is intended to mean something other than what we have meant, then its meaning is not intelligible.’”