I am wondering how justice fits with traditional definitions of natural law.  Aquinas mentions five fundamental natural law precepts, namely life, the procreative union of male and female, the education of the young, living in society, and the worship of God.  Most natural law thinkers would also state that justice is a key principle of the natural law.  However, it does not seem to me to be clearly stated within Aquinas’s fundamental precepts or to fit neatly with them.


Very good question, but I think you have this difficulty with St. Thomas only because of a misreading – a common one.

In the place in the Summa theologiae I think you have in mind, he says that the order of the precepts of natural law corresponds to the order of the natural inclinations, and he speaks of three inclinations – the inclination we share with all substances, the inclination we share with all animals, and the inclination which pertains more particularly to our rationality.  You would think, then, that there should be only three fundamental precepts.  The reason there are more than three is that his term “inclination” refers not to particular inclinations, as we would use the word “inclination,” but to broad categories of inclinations.

These categories are pretty broad.  In speaking of what we share with animals, for example, he gives two examples -- sexual intercourse and the education of offspring, as you note – but then he says “and so forth.”  Rightly so, because we share a great many other things with other animals too, such as learning from sense impressions.  Then, in speaking of our rationality, he mentions the inclination to seek the truth, especially the truth about God, as well as the inclination to live in society.  Although this time he doesn’t say “and so forth,” once again these are just examples; they don’t exhaust the set of inclinations we have because of rationality.  For instance, only a rational creature seeks beauty and tries to be witty.

To back up for a moment, it’s pretty obvious how the inclination to know the truth is connected with rationality.  But how is the inclination live in society connected with it?  After all, don’t cows live in herds, and don’t birds live in flocks?  And they certainly aren’t rational.  Upon reflection, though, one realizes that what St. Thomas means here by society isn’t just how cows and birds hang around together, but partnership in a life shaped by truth and rational principle, something the beasts can’t enjoy.

And now things really get interesting, because among the rational principles essential to such a partnership is justice, which is equal treatment of those who are equal in the relevant respects.  So yes, we do have a natural inclination to seek justice -- and doing so is a fundamental precept of natural law.

My correspondent had continued:

I suppose there are two possibilities: one, that Aquinas’s definition needs to be supplemented by an additional precept, which leaves open the possibility that there are additional primary precepts of the natural law. I can imagine that this is an unattractive prospect for someone in the Thomist tradition!  


As you now know, this Thomist thinks there are lots of other primary precepts -- and that the prospect is not at all unattractive! 

To avoid getting St. Thomas wrong, it’s important to keep in mind his fondness for the figure of speech called metonymy:  He often uses the most conspicuous instances of things as placeholders for entire sets of things, and this is what he is doing when he speaks of our inclinations.

My correspondent had again continued:

Another possibility is that justice is inherently related to the precept about human sociability.  Because we have an inherent need to live in society, and we ought to do to others what we would have done to us, it follows that we ought to give every person his due.  From this we could presumably deduce principles about impartiality, procedural fairness, etc. 


Right!  And you know I agree, because your remark anticipates what I wrote a few lines above.


Commentary on Thomas Aquinas's Treatise on Law