There's been a certain amount of fuss lately over the fact that the five Supreme Court justices in the majority of the recent Carhart ruling are all Catholic. Certain commentators have declared themselves shocked, shocked that at the possibility that some or all of those justices may be allowing their personal beliefs as Catholics to interfere with their ability to rule from the bench. (With Justice Kennedy this would be a welcome change, but hardly seems to be a sure thing.)
What this raises, or course, is the question of: What exactly is a judge supposed to use as his frame of reference when forming a court ruling?
Clearly, we are a nation of laws, and as such (call me a positivist but there it is) a judge is first and foremost required to rule based on what the law says. But however much one wishes to stick to the idea of positive law as the basis for civil proceedings, clearly a judge cannot rule on the basis of positive law alone.
It is quite simply impossible to define every aspect of a situation. Thus, positive law is invariably interpreted through the lens of the judge's assumptions about the world, whether those assumptions be derived from religious faith, philosophical conviction, or emotional or ideological appeal.
If there were to be a sudden wave of hate against red-headed people and congress rushed through provisions allowing for the confiscation of their property, their imprisonment without trial, etc. the courts would be faced with the task of determining whether the protections laid out in our constitution were meant to apply to red-heads, or only to people with other hair colors. The constitution is silent on this point, since there has never in reality been a necessity of defining that rights apply to all hair colors, and yet this very silence (resulting from previous agreement) would mean that the decision of whether or not such protections applied to red-heads would effectively be left to judges and whatever philosophical or theological guidance they brought to the table in making that decision.
This is where any real law system simply cannot be ideal. For instance, there is I think a certain sense in which passing constitutional amendments that abolished race-slavery and banned racial discrimination was an incorrect approach. If one truly holds that the rights of members of all races should be equal, you shouldn't have to specifically "give" rights to one race via an amendment. They already have them by virtue of their humanity.
And yet, at a practical level, so long as there were a number of members of the judiciary to preferred not to acknowledge that all humans had the same rights, it was necessary to set our this equality by means of positive law in order to avoid a situation where different judges handed down opposite decisions based upon their own ideas of natural law.
Thus, things which ought properly simply be understood as a matter of natural law (such as who is a human being -- and thus deserving of human rights) often end up having to be defined by means of positive law if they come into sufficient question within society. However, we should understand that this is a crutch, not an ideal state. It is not the case that every group right down to the individual must be specifically defined in order to be deserving of their rights under the law. Rather, our need to define such things is the result of the inability of society to clearly understand and obey natural law.