IIIc. Particular Precepts as Not Known
The recognition that people do not perfectly know the natural law has been an important part of natural law theory from the beginning, and almost any major discussion of it spends some time talking about why people deviate from it. From everything we have discussed so far, it is clear that (1) the first precepts of natural law are the first principles of practical reason insofar as they concern common good; (2) the natural law is common to all human beings. It does not, however, follow from this that natural law theory is committed to the claim that everyone has equal knowledge of the precepts of natural law. People can fail to recognize somethings as belonging to natural law.
In a sense, we shouldn't be surprised by this anyway. The first principles of logic are common to all human beings, too, but nobody thinks that this implies that people have a perfect understanding of logical principles and their implications, nor does anyone think that it follows that people will be perfectly logical. However, we should expect this variation to an even greater extent in the case of natural law, for several reasons.
(1) Practical matters depend heavily on circumstantial details. Theoretical discussions can abstract from details fairly easily, but when we are discussing practical matters, the details will often matter. Subtly different details in our circumstances will sometimes require very different rational responses. You and I could be in apparently similar situations, judge differently, and both be judging correctly given some small but significant difference in detail between our situations. It does not follow, of course, that all differences are significant in this way, but some can be. The more general we are, the less of a problem this is, but staying at a purely general level in practical judgment is not a very practical procedure. We need to get to particular conclusions, and these can be very circumstance-dependent.
(2) Conclusions are harder to know than principles. By definition, a conclusion is something arrived at by reasoning, and reasoning often takes work. Extensive chains of reasoning can be difficult to follow and increases the possibility of error, which will sometimes require special steps to avoid. This, of course, is true in general, and not a purely practical matter, but, especially given the detail-oriented character of practical reasoning, this is one reason why so much practical judgment is by "eyeballing" or estimation. In practice we often don't reason out whether things are right or wrong so much as see whether things look at first glance like they fit our general template of right or wrong, whether they "sound right", because this saves time and effort.
(3) Bias and prejudice are especially likely to affect practical reasoning. Biases and prejudices can certain affect speculative reasoning, but this is nothing in comparison with what they sometimes do in practical matters. As Aquinas notes (ST 2-1.94.4), our reasoning in these cases can be thrown off by "passion, or bad custom, or bad disposition of nature". Strong attachments or revulsions can affect our practical assessments; education and the behaviors of people around us can affect them; and character traits can certainly affect them. Thus it is not surprising that people can come to the wrong conclusions when these are involved and are in some way inappropriate. In any of these ways people can fail to know the more intricate precepts of natural law. It's clear enough, too, that this fits our experience: practical matters about which people get very emotional, or which involve questions of whether or not to throw off well-established customs, or in which people can easily be motivated by various sorts of excesses or deficiencies of character, are often very contentious.
These three are mentioned by Aquinas. It's an interesting question whether they are exhaustive. Given their generality they perhaps can be considered so, as categories; but it does seem clear that there are common specific patterns of error. For instance, people often confuse private good with common good and vice versa. This directly affects natural law, because the principles of practical reason are only law to the extent they concern common good -- for instance, our common good as existing, living, reasoning beings. This is why practical understanding of, say, painting, is not itself part of natural law (although governed by it at least indirectly): the goods we deal with in painting this or that are not goods shared in common in the way we all share in common the good of people striving to be rational, trying to live in peace with each other, respecting property at least to some reasonable extent, educating their children and generally working to leave the next generation better off, being allowed to defend themselves in moderate ways, doing the kind of work required to sustain the human race, etc. It's equally a sort of practical reasoning, and is based on the same principles; but the practical rules of painting are not based on those principles as law. It's not difficult to recognize, however, that people are constantly slipping back and forth between practical reasoning concerning their own (or someone else's) private good and some kind of common good. All three of the above factors are certainly involved in this. Circumstantial details can matter in drawing the line between private and common good. When we're dealing with borderline cases, it can be difficult to be precise enough about what common goods we're considering to handle them solely in light of common good. (There is no doubt that people often estimate what common good must be based on what benefits themselves. Indeed, this is often an entirely reasonable thing to do, when people have a sufficiently enlightened and informed understanding of their own interests.) But even more than this, biases and prejudices are constantly leading us to favor our own good over what is good for everyone, often in subtle ways that we do not notice unless we are really looking. This kind of error, while perhaps traceable to the three categories mentioned above, appears to be a hybrid of them all.
The really worrisome part is the fact that vice can interfere with our practical judgment. There's a sense in which every vice is self-justifying: by being a stable disposition of character inclining for or against something, it inevitably affects our judgment, and begins setting up a sort of anti-prudence. (Historically this 'anti-prudence', which varies depending on the vice, is called a 'daughter vice'.) This is one of the reasons why natural law theorists have historically put immense emphasis on the importance of (1) careful and deliberation cultivation of prudence, (2) formation of conscience, (3) well-ordered positive laws, and, in Christian versions, (4) divine grace.
As I said before, the causes of deviation from natural law are quite important to natural law theory, and in practice most of what natural law theorists do is not deduce conclusions from first principles but work backwards by looking at what conclusions show signs of being vice-friendly. There are advantages and disadvantages to this way of proceeding. One disadvantage is that it never actually gets you beyond probable arguments and presumptive reasoning. Another disadvantage is that it can lead to a failure to take some special circumstance into account. On the other hand, the difficulty of rigorous argument in practical matters should not be underestimated, and no matter how apparently rigorous your deductions, you will at some point have to consider very seriously the possible sources of error in your reasoning. In reality, serious discussion of natural law requires careful consideration from both ends: both an attempt to apply general principles rigorously and an attempt to pin down any particular sources of error. In some cases this is not difficult at all, but there will always be cases where it will be extraordinarily difficult. But one of the key features of natural law theory is its insistence that inquiry into ethical matters is fundamentally a rational inquiry in which the entire human race is involved, and that this inquiry is not a free-for-all but actually capable of rational progress.
Having discussed the particular precepts of natural law, we can now move on to the last major element of natural law theory, namely, its connection to positive law, which we will do in a future post.