Summa Theologiae Moralis, vol. I: De Principiis

233. The proper effect of law is obligation. It holds a middle among the various effects of law : the ultimate effect on the part of the legislator is obligation taken actively, or the force of binding which resides in the law, to which in the subjects, as the first effect, there corresponds the obligation taken passively, i.e. the very moral bond and necessity of obeying.

Now obligation is a moral necessity of doing or omitting something, categorical or imperative. It is called

1. moral or objective necessity, which belongs to a free being and by which something is necessary by a necessity of end (as a means or precept), so that without it the end is not obtained. Hence it is a moral bond, or practical necessity of choosing a means to an end, whereby on the supposition of the end such a means must necessarily be exhibited ; and so it is distinguished from a physical bond or physical necessity of subjective efficiency, both natural, which takes away liberty, and coercion, which also destroys the will ;

2. categorical or imperative, i.e. not only conditional, on the supposition of the end if one wishes to obtain it, but absolute, by which also the end itself is imposed. Hence obligation arises from a twofold necessity : from the absolute necessity of the act with respect to the ultimate end, and from the absolute necessity of this end. And so it is shown how it does not harm liberty : it binds the will inasmuch as the intellect proposes the end and the means, leaving intact the physical power of acting or not acting¹.

For obligation imports a twofold debt : one according to itself, and so the end is due ; another for the sake of something else, i.e. by which it is impossible to reach the end. And so « in debt is the order of exigence or necessity of something to which it is ordered » (I, q. 21, a. 1, ad 3).

Hence obligation, like morality, is an intrinsic habit and real order, or a transcendental relation :

a) in the obligating act, just as morality is a real order of conformity or non-conformity, so obligation is a real order of necessity to the object and the ultimate end (I-II, q. 99, a. 1) ;

b) in the obligated person, it is not only a relation of subjection to the person of the legislator, which is presupposed, but it adds a new relation by which he is subjected as to this or that prescribed thing (II-II, q. 16, a. 1).

Obligation therefore is the property of a human act according to which it is said to be necessarily to be posited or avoided for the ultimate end.

234. Division of law by reason of diverse obligation. — Obligation is diverse according as the law has directive or coercive force, and therefore by reason of the effect or of diverse obligation, one law is moral or preceptive which only directs and obliges to positing or omitting the act, another penal which only punishes him who omits the prescribed act or commits the forbidden one, another mixed which does both. Every law obliges in conscience to something which the legislator intends, otherwise it would not be a true law but a counsel ; but this is the difference between a purely preceptive and a purely penal law, that the first only obliges to positing or omitting the act established by law without inflicting a penalty on transgressors, namely : everyone do this, e.g. hear Mass ; the second obliges only to undergoing the penalty which the law requires of him who has not posited or has omitted the act established, namely : whoever has done this, e.g. who has hunted or fished before this day, shall pay a sum : since in positive law the whole obligation depends on the legislator, just as he may not oblige, so he may oblige more or less, or to the penalty alone. From these is distinguished a mixed law, which is preceptive or moral and also penal, namely, no one do this under penalty : this obliges in conscience to positing or omitting the act and at the same time to undergoing the penalty if it has not been posited or omitted ; but a merely penal law does not oblige in conscience to positing the act, but under penalty alone, to which however it binds in conscience².

By reason of form, a moral law is distinguished into affirmative or prescriptive, which imposes something to be done, and negative or prohibitive, which imposes something to be omitted ; the former obliges always but not for always, i.e. for some time only while it lasts ; the second obliges always and for always, i.e. for every time while it lasts³. And surely at every time we are bound to abstain from every evil, but not to do every good.

To the preceptive is reduced the permissive law, which prescribes the liberty of someone and forbids that he be hindered by others ; to the penal rather is reduced the irritant law, which compels by invalidating the act placed against the law.