After every circumstance, every relation is known, the understanding has no further room to operate, nor any object on which it could employ itself. The approbation or blame which then ensues, cannot be the work of the judgement, but of the heart: and is not a speculative proposition or affirmation, but an active feeling or sentiment.
In this passage, Reason becomes absented from moral consideration and "an active feeling" assumes a primary role in ethical behavior. Understanding here seems to be a nearly photographic apparatus which totally and passively apprehends the immediate facts of a case, while sentiment acts. Feeling becomes both the judge and the capacity for judgement; the heart, a radiating moral center.