What does this quote from the beginning of chapter XVIII mean? Quote from Hawthorne about Dimmesdale's sin: "had been a sin of passion, not of principle, not even purpose"
This is part of Hester's view of what she and Dimmesdale have done. There are many types of "sin" but one of the major distinctions is between sins of passion and sins of the will (principle/purpose). We seem to be hard wired for many sins of passion, and our tendency toward promiscuity is one of these. Hester has an understanding that these sins are not so bad because they are not sins of the will. Compare this to Chillingworth's sin which is a sin of the will: destroying the life of another person for his own purposes.
We are not, Hester, the worst sinners in the world. There is one worse than even the polluted priest! That old man's revenge has been blacker than my sin. He has violated, in cold blood, the sanctity of a human heart. Thou and I, Hester, never did so!"
This quote comes in Chapter 17, right before Hester's comment and highlights the difference in the two types of sin. Premeditated acts, acts of the will such as revenge, are much more sinful that acts of passion.