Finnis on the relationship between judicial impartiality and the technical rationality of positive law

I recently came across this passage from John Finnis that explains the relationship between judicial impartiality and the technical rationality of the law:

In the working of the legal process, much turns on the principle--a principle of fairness--that litigants (and others involved in the process) should be treated by judges (and others with power to decide) impartially, in the sense that they are as nearly as possible to be treated by each judge as they would be treated by every other judge. It is this above all, I believe, that drives the law towards the artificial, the techne rationality of  laying down and following a set of positive norms identifiable as far as possible simply by their 'sources' (i.e. by the fact of their enactment or other constitutive event) and applied so far as possible according to their publicly stipulated meaning, itself elucidated with as little as possible appeal to considerations which, because not controlled by facts about sources (constitutive events), are inherently likely to be appealed to differently by different judges. This drive to insulate legal from moral reasoning can never, however, be complete.

John Finnis, Natural Law and Legal Reasoning, in Natural Law Theory, Robert George, ed., p. 150.

While Professor Finnis acknowledges that legal reasoning is never completely insulated from moral reasoning, this passage explains one way in which natural law theory justifies the positivity of positive law. It is a helpful corrective to a tendency in contemporary constitutional theory to set natural law reasoning in opposition to constitutional originalism.