It is that proposition whose judgment concerning its subject, in both external reality and mental conception, constitutes a primary, not a secondary, predication—that is, it is affirmed without the mediation of anything else. Whenever the predicate is attached to a subject only through something intermediary, the predicate of that intermediary must first be affirmed; in that case the judgment is no longer primary but secondary.

This difference becomes apparent only when the intellect has no motive for its judgment other than the mere presence of the two terms—subject and predicate. If, however, other factors intervene—tender‐heartedness, pride, tribal zeal, habit, approval of the universal order, or concern for the common good—then one cannot tell whether the intellect’s assent is due solely to the presence of the two terms or to one of those extraneous causes.

If you wish to test a proposition, imagine yourself as though you had just been created in that very state, paying no heed to custom, to the rulings of the other faculties such as compassion and tenderness, and stripping away everything to which you have grown accustomed, along with all considerations of expediency. Then present to your mind the two terms, subject and predicate. If their mere presence compels the intellect to affirm the relation, the proposition is a first principle; otherwise it is merely well-known. Thus we say, ‘Lying is reprehensible,’ whose notoriety stems from its link to the common good; or we say, ‘Inflicting pain is reprehensible,’ whose acceptance is rooted in tenderness. The proof that these are not first principles is that, under the foregoing thought-experiment, when we present to the intellect ‘A thing is never devoid of affirmation or negation’ and also present ‘Lying is reprehensible,’ we find the intellect decisively assents to the former while it hesitates over the latter; hence it is not among the primary certitudes.

A well-known proposition may be true or false. When true, it may indeed be a first principle, yet it may also require demonstration. Every first principle is necessarily well-known, but not every well-known proposition is a first principle. A proposition gains its notoriety either because it is primary, or because it is tied to social order, or because of psychological impulses, as we have mentioned, or because of broad induction; for every school of thought possesses notions well-known among its adherents that perhaps are not