Suppose that a previously existing capacity is gradually weakened. The question becomes when the deed could occur through it. If action requires sufficient power, why should an earlier weaker capacity be regarded as a complete productive cause?
This is why the earlier capacity may be described as a condition or potential readiness rather than being equated without qualification with the particular power through which the deed actually takes place.
The key distinction is between an enabling condition existing in advance and a determinate relation of power to an act as it occurs.