known and given relations of the parts of these figures; and thence infers some unknown relation, which is dependent on the former.  But in moral deliberations we must be acquainted beforehand with all the objects, and all their relations to each other; and from a comparison of the whole, fix our choice or approbation.  No new fact to be ascertained; no new relation to be discovered.  All the circumstances of the case are supposed to be laid before us, ere we can fix any sentence of blame or approbation.  If any material circumstance be yet unknown or doubtful, we must first employ our inquiry or intellectual faculties to assure us of it; and must suspend for a time all moral decision or sentiment.  While we are ignorant whether a man were aggressor or not, how can we determine whether the person who killed him be criminal or innocent?  But 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 the disquisitions of the understanding, from known circumstances and relations, we infer some new and unknown.  In moral decisions, all the circumstances and relations must be previously known; and the mind, from the contemplation of the whole, feels some new impression of affection or disgust, esteem or contempt, approbation or blame.

Hence the great difference between a mistake of fact and one of right; and hence the reason why the one is commonly criminal and not the other.  When Oedipus killed Laius, he was ignorant of the relation, and from circumstances, innocent and involuntary, formed erroneous opinions concerning the action which he committed.  But when Nero killed Agrippina, all the relations between himself and the person, and all the circumstances of the fact, were previously known to him; but the motive of revenge, or fear, or interest, prevailed in his savage heart over the sentiments of duty and humanity.  And when we express that detestation against him to which he himself, in a little time, became insensible, it is not that we see any relations, of which he was ignorant; but that, for the rectitude of our disposition, we feel sentiments against which he was hardened from flattery and a long perseverance in the most enormous crimes.

In these sentiments then, not in a discovery of relations of any kind, do all moral determinations consist.  Before we can pretend to form any decision of this kind, everything must be known and ascertained on the side of the object or action.  Nothing remains but to feel, on our part, some sentiment of blame or approbation; whence we pronounce the action criminal or virtuous.