Summa summae S. Thomae, Tomus II

I. Interpretation generally taken is the explanation of an obscure law, whereby it is declared whether in such a case it obliges or does not oblige. It is twofold: one verbal, which is called simply interpretation, and is the explanation of words conformably to the mind which the legislator had when enacting the law; the other, which is called epiikia, is the explanation of the mind, not which the legislator had when enacting the law, but which he would have had if he had attended to the present case, and would have, if he were interrogated here and now.

[II. Simple interpretation is authentic, usual and doctrinal.

Authentic interpretation, or that pronounced with authority, is that given by the legislator himself, or by his successor or superior. But whether such interpretation ought to be promulgated in order to oblige? A distinction must be made. Either it is purely such, or it is a mere declaration, and promulgation is not required, since it is not a new law: it binds those to whom it de facto becomes known, inasmuch as it determines the sense of the law, and thus the law must be observed. Or it is not purely such, that is, it somewhat changes the received sense of the law; and then, since at least in part it is a new law, it needs promulgation.

Usual interpretation is sought from custom, whereby, as said in cap. Cum dilectus, Decretal. lib. I, tit. 4, de consuetudine, « it is the best interpreter of laws. » Custom interprets the law when it is at least in part according to the law, which it can also in part change; for, if it be against, it abrogates the law, and if beside [the law], it induces a new law.

Doctrinal interpretation is made through the glosses of theologians, canonists and jurists. Of itself it has not the force of law, since it only expounds the sense of the law, and doctors have not legislative power. However, if an interpretation commonly admitted deviate more or less from the pristine sense of the law, herein it introduces new right, and under this respect becomes usual.]

III. In simple interpretation of laws the proper, common and more received sense of words is to be kept unless some inconvenience follow; because by law it is presumed that the legislator spoke in this sense.

A favorable law harming no one must be amplified; an odious one restricted, according to rule of law 16 in 6°.

In human laws argument from parity is not regularly made: because in things depending on free will there is no conclusion from parity; but the obligation of human law depends not so much upon the reason of the law as upon the free will of the legislator, which was about that case and not about this, although similar. Otherwise in natural law, on account of the opposite reason: because its obligation is from the nature of the thing and from reason itself, which while fighting for one case fights equally for a similar one.

I said, « regularly; » because in correlatives, as husband and wife, in things equilibrated by law, as the executor and the mandator, and in those whereof one is subordinated to the other as imperfect to perfect, as eggs to flesh, with respect to him to whom permission to eat flesh is granted, argument from similitude is made.

IV. As to epiikia, it is certain that it likewise has place in human laws ¹ ; because laws are enacted generally for the common utility and according to what happens in the majority of cases, nor can the legislator foresee and except all cases. Yet it often happens that something whose observance is useful to the common good in most cases is nevertheless harmful in some cases. Whence, if a case emerge in which literal observance of the law is harmful to the common good, or brings notable detriment to the observer or another in health, reputation or fortune — for this redounds against the common good — or be contrary to natural law or higher human law, or become unjust or too hard through some circumstance, then per epiikia it is not to be observed; because it is deservedly presumed not to have been the legislator's mind that the law oblige in these cases.

You will ask whether it be likewise lawful to use epiikia in laws invalidating contracts.

Resp. More probably negatively, on account of the grave inconveniences which would thence arise against public good, and which prevail over the evil, howsoever grave, of some particular person. For if it were permitted to use epiikia in an invalidating law, e.g., clandestine marriage, contracts of minors, or made without tabellion, concubinage and frauds would rage everywhere to the ruin of the republic. Add that a contract does not stand without the form prescribed by those laws.