ANSWER: (1) A lack of strength to endure the temptations and trouble of this life; (2) a consciousness of weakness which caused Paul to ally himself with Christ so that he could say "when I am weak then am I strong." "An infirmity," says Fletcher, "is consistent with pure love to God and man; but a sin is inconsistent with love. An infirmity is free from guilt, and has its root in our animal frame; but a sin is attended with guilt and in our moral frame, springing either from the habitual corruption of our hearts, or from the momentary perversion of our tempers. An infirmity has its foundation in an involuntary want of power; and a sin is a willful use of the present light and power we have."
— Steele's Answers p. 134.