From Martin Luther, Commentary on Galatians, 510-514, quoted in William James, The Varieties of Religious Experience at 115:
When I was a monk, I thought that I was utterly cast away, if at any time I felt the lust of the flesh: that is to say, if I felt any evil motion, fleshly lust, wrath, hatred or envy against my brother. I assayed many ways to help to quiet my conscience, but it would not be; for the concupiscence and lust of my flesh did always return, so that I could not rest, but was continually vexed with these thoughts: this or that sin thou has committed: thou art infected with envy…, But if then I had rightly understood these sentences of Paul ‘The flesh lusteth contrary to the spirit…,’ I should not have so miserably tormented myself, but should have thought and said to myself, as now commonly I do, ‘Martin, thou shalt not utterly be without sin, for thou has flesh; thou shalt therefore feel the battle thereof.’ I remember that Staupitz [Luther’s spiritual mentor] was wont to say, “I have vowed unto God above a thousand times that I would become a better man: but I never performed that which I vowed. Hereafter I will make no such vow; for I have now learned by experience that I am not able to perform it. Unless, therefore, God be favorable and merciful unto me for Christ’s sake, I shall not be able, with all my vows and all my good deeds, to stand before him.