In Summa Theologica II-I Q96 A2 corp. Aquinas says “a measure should be homogeneous with that which it measures”. While I could gather roughly what he was saying from the context, I must admit that this phrase confused me a bit. But what he’s saying isn’t really that confusing or complicated when we consider common examples of measures.

For instance, a ruler can’t measure length unless it too has length, and a clock can’t measure duration unless it persists through some duration. So that’s the first sense in which a measure is homogeneous with that which it measures: it must share the relevant characteristics of that which it measures.

We can take this further. A 30-centimeter ruler is not well-suited to measuring kilometers or nanometers, but it is well-suited to measuring many everyday household objects and regular sized drawings. Similarly, a clock that measures in seconds is not well-suited to measuring nanoseconds or hours. This raises a second sense in which a measure is homogeneous with that which it measures: it must be of a well-suited “scale”.

It seems to me that in the article, Aquinas is primarily concerned with the second sense mentioned here. “Law”, he says, “is framed as a rule or measure of human acts.” That is, the law of a community encodes what behaviour is good, and so it is by the requirements of that law that we judge to what extent actions are good or bad. Now, just as the length of a ruler should be scaled to the lengths we seek to measure, so “laws imposed on men should also be in keeping with their condition.” It is on account of this that even though an ideal law might forbid all vices, practically this isn’t a good idea:

Now human law is framed for a number of human beings, the majority of whom are not perfect in virtue. Wherefore human laws do not forbid all vices, from which the virtuous abstain, but only the more grievous vices, from which it is possible for the majority to abstain; and chiefly those that are to the hurt of others, without the prohibition of which human society could not be maintained: thus human law prohibits murder, theft and such like.