I promised reader Ficino that I would write this post. It came up because of an exchange that I've heard a few different times, on different subjects. It goes like this:
Atheist: You shouldn't try to enforce your religion by law!
Catholic: Ah, but that's not my religion I'm enforcing by law! It's my Thomistic philosophy. Since philosophy is rationally provable, that makes it nonreligious and therefore okay to enshrine in the law.
Most non-Catholics find this argument completely ridiculous because Thomistic philosophy isn't something they're familiar with in the first place, and it sounds suspiciously like a religion. I would argue that Thomistic philosophy is, in fact, indistinguishable from a religion.
For instance, the claim that this philosophy is rationally provable is something that many religions also claim. Catholics especially. They believe that is it possible for unaided reason to know the existence of God. From there, I am not sure belief in the Catholic Church itself is supposed to be rationally provable, but it's at any rate assumed to be likely.
Like a religion, Thomistic philosophy is not actually provable. I've gone back and forth with Thomists and what it usually comes down to is something like this: "We can prove xyz from first principles, but the first principles themselves you just have to accept. But you have to accept something without any proof, something to base later beliefs on, and we think our first principles are reasonable to accept."
The trouble is, this argument could be used for almost anything. You have to accept something without proof, to base later beliefs on . . . so why couldn't this first belief be that the bible is divinely inspired? Or that Joseph Smith found golden tablets? Or that Muhammad was visited by an angel? What makes "all things have a final cause" any different from these premises?
Now it's true that we have to make moral judgments in order to decide what should be enshrined in law. To some degree, it doesn't make sense to criticize "legislating morality," because that's what we all do. Yet there are some versions of morality we don't want to legislate. Some people believe that polygamy or child marriage are moral. And the rest of us say, "We don't want that, because it's strictly part of your religion." But is that really why we don't want it? I'd say we don't want it because it's against our own morals.
Personally, I believe in an Enlightenment morality based on rights, such as life, liberty, and the pursuit of happiness. And this is the moral code enshrined in our country's founding documents. Can I prove this morality is better than natural law? Perhaps not. Especially not to people who have an attachment to natural law which stems from religion. (Because people normally come to a belief in natural law because they are already Catholic. It's pretty rare for it to happen the other way around.) But neither can you prove to me, or to a majority of voters, that natural law is better. You can say your system is provable, but that doesn't make it so. To me the claim that "if you read this long book by Ed Feser, you'll understand and accept the proofs" is no different from "if you read the Book of Mormon, you'll feel a burning in the bosom and know that it's true." It's a big claim, and yet in both cases you can find people who have read the books and not been convinced at all. So it's possible that there is an error in the proofs. In any event, the jury is out on whether your metaphysics is actually rational and provable.
The real issue is that the dichotomy between "Catholic morality that comes from natural law" and "Catholic morality that comes from Scripture" is artificial. Catholics don't actually see them as different (all are obligatory) and non-Catholics don't see them as different (all are religious). And Aquinas himself, working this stuff out, ruled out any conclusions of natural law that might be opposed to Scripture or Catholic teaching of his time.
And Catholic teaching itself never says that those moral laws that are known by faith shouldn't be enforced by law, while those known by reason should be. The traditional teaching of the Church is that the existence of God is known by reason. Therefore it would follow that it could be a crime to fail to worship God. The modern Church has come out in favor of freedom of conscience, but as little as a hundred and fifty years ago, it taught that error has no rights and that it is legitimate to establish a religion or criminalize heresy. And honesty, this view is more consistent with the inital assumption that anything a Catholic thinks is provable by reason can be legislated. Yet I doubt the people arguing that it's valid to (for example) ban homosexuality on natural-law grounds would like to come out and say that what they want is a Catholic theocracy. It's likely that they don't want that, but according to their arguments, I can't see why not.
Those of us who aren't Catholic may do better to simply say that unless a law can be defended from the first principles in our founding documents, it isn't valid. As a nation, we already have a basic moral philosophy. It might be very general, so that complex arguments may be required to lead from there to any specific law, but it has the advantage of having already been accepted by the nation at large.
A small, religious minority who feels they have a better answer, who would like to legislate not based on life, liberty, and happiness, but on beliefs that are not general to non-members, should in my opinion stick to making rules for their own members. I think that Catholics would acknowledge this if we were talking about, say, Muslims. And they wouldn't at all buy the notion that a certain rule wasn't from the Quran, but rather worked out by Averroes from pure reason. If it's really reasonable, the argument will have to be made by appealing to life, liberty, and happiness. If it can't be--perhaps it's reasonable only based on first principles the rest of the country doesn't accept. And in that case, it's indistinguishable from a religious argument.