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What happens when one spouse changes his or her mind about forgiveness?

This sometimes happens in cases where one spouse is repeatedly unfaithful to the other.

This is the revival principle. If the spouse cannot forgive, the grounds are said to be revived, and the defense of condonation is no more. Courts used to require evidence of "conjugal unkindness" as a precedent to invoking the revival principle. As one might imagine, the definition of "conjugal unkindness" was a bit vague.