"Law is a rule and measure of human acts, whereby man is induced to act or is restrained from acting". I II 90.1. This brief explanation of the essence of law, which will be repeatedly refered to throughout the Treatise on Law, contains the important concept "rule and measure".

The definition of law which is most commonly quoted is found in Article 4:

puts emphasis on common good.

Let's look at the two concepts: "rule and measure" and "common good".

Rule and Measure

A simple definition of "rule and measure" might be "to govern" but let's go a bit further because the concept "measure" includes the way we know and turns out to be quite important for our topic. Josef Pieper points us to a helpful passage of St. Thomas from his Disputed Questions Regarding Truth:

Created things, from which our intellect receives knowledge, give the measure to our intellect. But they have received their measure from the divine intellect, in which all created things are as all objects of art are in the mind of the artist. Thus, the divine intellect gives the measure and does not receive the measure. But created things both give and receive the measure. But our intellect, in regard to natural objects, is receptive of the measure and does not give the measure. It does this only in regard to artifacts. Ver I,2

Pieper continues

Mensura and mensuratum, that which provides the measure and that which receives the measure, differ only in their different positions in the order of importance and meaning involved in their realization. That which provides the measure is, in its quality of medel and original form, identical with the recipient of the measure. That which receives the measure is, in its quality of image and copy, the very measure itself. Our knowledge, then, as image and copy, is reality itself.

Josef Pieper, Living the Truth, Ignatius Press, p. 125

More from St. Thomas on rule and measure:

Natural things are midway between the knowledge of God and our knowledge: for we receive knowledge from natural things, of which God is the cause by His knowledge. Hence, as the natural objects of knowledge are prior to our knowledge, and are its measure, so, the knowledge of God is prior to natural things, and is the measure of them, as, for instance, a house is midway between the knowledge of the builder who made it and the knowledge of the one who gathers his knowledge of the house from the house already built. I 14, 8 ad 3

The first article continues: The "rule and measure of human acts is the reason which is the first principle of human acts" i.e. reason directs human acts to their end. Irrational creatures are moved to an end which is fixed by their natural appetite. Man is different. His reason directs his acts to his end and law is a "rule and measure" of those acts. Therefore, law cooperates with reason in determining how to act. (I II 91,2 ad 2)

For the Common Good: The first interesting point in Thomas' discussion on this topic is that law is related to happiness, the happiness of the individual. This is perhaps an unexpected association for the modern reader. It follows from the fact that the first principle of the practical reason is the end/purpose of the individual's life quest which St. Thomas earlier in the Summa concludes is the enjoyment of God (bliss/happiness). Since law is related to practical reason and practical reason directs man to his end which is happiness, law is related to (aimed at, directs man toward) man's happiness.

Law is related to (the individual's) happiness and since "every part is ordained to the whole", and each man is a part of a community, it follows that "law must relate to the happiness of the entire community".

What is this Common Good? Thomas does not elaborate in essay like fashion. He states that common good is twofold: a sensible and earthly good and an intelligible and heavenly good. (I II 91,5).

For Aquinas, the human person is always simultaneously for his own sake and for the sake of another, because acting for the sake of another is perfective of human nature. Aquinas's notion of the common good precludes the modern opposition between individualism and universalism by proposing that each and every person is constitutive of the common good and by holding that the common good is an instrument for achieving the end of each person within society. Where liberalism sees the common good as either a useful means for the invidividual's pursuit of happiness or a universalist aim for which the individual must sacrifice his personal preferences, Aquinas considers the common good as at once useful and for its own sake. For Aquinas's anthropology, individual happiness requires participative sharing in the good of others. In contrast with liberals who consider the good common simply insofar as many partake in it, Aquinas considers the good as good precisely insofar as it is enjoyed in common. Where the liberal sees sharing as a dividing up, Aquinas sees it as a sort of festive feedback in which the more the others enjoy the more I enjoy.

Aquinas carefully distinguishes between the private good of the individual and the common good of the political community. Whereas each creature is always entirely for God, the human is not entirely for the sake of the political community. The political common good and the individual\s good only partially overlap. And therefore, part of the individual's good is not contained within the good of the political community.

Robert Gahl, unpublished paper, "A Critical History of Common Good".

This concept of the common good relies on the assumption that man has a nature which is identical for all and, thus, an end which is common to all. (I II 1,7). Actions (and the practical reason) are concerned with particular matters but these can be referred to the common good as to a "common final cause" (purpose), "as the common good is said to be the common end." (I II 90.2 ad 2)