Some of the later writers on international law have treated the doctrine of an universal law of nations, founded on the common agreement of mankind, as an empty fiction, to which nothing in fact really corresponds. But it never was intended by Grotius, to set up a rule like that which theologians have termed the golden rule of Vincentius Lirinensis, “Quod semper, et ubique, et ab omnibus.” The words of Grotius are, “there are two ways of investigating the Law of Nations. We ascertain this law either by arguing from the nature and circumstances of mankind, or by observing what is generally approved by all nations, at least by all civilised nations. The former is the more certain of the two; but the latter will lead us, if not with the same certainty, yet with a high degree of probability to the knowledge of this law. For such an universal approbation must arise from some universal principle; and this principle can be nothing else than the common sense or reason of mankind. (L. 1. c. 1. § 3.)
Grotius might have gone a step further, and might have said that this common consent of mankind was the voice of God declaring his will through the common conscience of the human race. It is thus indeed that many things which upon a priori reasoning might be justified by the Law of Nature as strictly deducible from admitted first principles, are condemned before the tribunal of the human conscience; and when the agreement of all the more civilised nations in such matters is ascertained, we are not at liberty to disregard it consistently with moral duty, nor may we safely fall back within the domain of abstract principle.
