Ok, I read that page and then by doing a search I found "Chapter 3, Institutes"
This is the answer:
Fifthly, that the direction of the will to good, and its constancy after being so directed, depend entirely on the will of God, and not on any human merit. Thus the will (free will, if you choose to call it so), which is left to man, is, as he in another place (Ep. 46) describes it, a will which can neither be turned to God, nor continue in God, unless by grace; a will which, whatever its ability may be, derives all that ability from grace."
I did see where the idea of "subsequent grace" could be traced back to Pelagius. As I understand it, Pelagius believed than man, through free will cooperating with God, could be perfected which is false.
So logically, the P follows from I, irresistible grace.