Summa theologiae IIaIae 90: The essence of law

Here there are four things to consider:

1. Whether law is something belonging to reason

2. The end of law

3. Its cause

4. Its promulgation

articulus 1: Whether law is something belonging to reason

It seems that law is not something belonging to reason.

obiectio 1: For the Apostle says (Romans 7:23): ‘I see another law in my members’, etc. But nothing which belongs to reason is ‘in’ the members, for reason does not make use of any corporeal organ. Therefore law is not something belonging to reason.

obiectio 2: Moreover, there is nothing in reason but power, habit, and act. But law is not a power of reason; similarly, it is not some habit of reason, because the habits of reason are the intellectual virtues, which have been spoken of above; nor again is it an act of reason: if it were, law would cease when the activity of reason ceases; for example, when we are asleep. Therefore law is not something belonging to reason.

obiectio 3: Moreover, law moves those subject to it to act rightly. But, properly speaking, it is the function of will to move someone to act, as is shown by what has been said above. Therefore law belongs not to reason, but to will; and this is in accordance with what the Jurist says: ‘What pleases the prince has the force of law.’

sed contra: It is a function of law to command and prohibit. But to command pertains to reason, as noted above. Therefore law is something belonging to reason.