It thus appears that we must recognise at least two principal meanings in the word "nature". In one sense, it means all the powers existing in either the outer or the inner world and everything which takes place by means of those powers. In another sense, it means, not everything which happens, but only what takes place without the agency, or without the voluntary and intentional agency, of man. This distinction is far from exhausting the ambiguities of the word; but it is the key to most of those on which important consequences depend.
Such, then, being the two principal senses of the word "nature," in which of these is it taken, or is it taken in either, when the word and its derivatives are used to convey ideas of commendation, approval, and even moral obligation ?
It has conveyed such ideas in all ages. Naturum sequi was the fundamental principle of morals in many of the most admired schools of philosophy. Among the ancients, especially in the declining period of ancient intellect and thought, it was the test to which all ethical doctrines were brought. The Stoics and the Epicureans, however irreconcilable in the rest of their systems, agreed in holding themselves bound to prove that their respective maxims of conduct were the dictates of nature. Under their influence the Roman jurists, when attempting to systematise jurisprudence, placed in the front of their exposition a certain Jus Naturale, "quad natura," as Justinian declares in the Institutes, "omnia animalia docuit ": and as the modern systematic writers, not only on law but on moral philosophy, have generally taken the Roman jurists for their models, treatises on the so-called Law of Nature have abounded; and references to this Law as a supreme rule and ultimate standard have pervaded literature. The writers on International Law have done more than any others to give currency to this style of ethical speculation; inasmuch as, having no positive law to write about, and yet being anxious to invest the most approved opinions respecting international morality with as much as they could of the authority of law, they endeavoured to find such an authority in Nature's imaginary code. The Christian theology during the period of its greatest ascendancy opposed some, though not a complete, hindrance to the modes of thought which erected Nature into the criterion of morals, inasmuch as, according to the creed of most denominations of Christians (though assuredly not of Christ), man is by nature wicked. But this very doctrine, by the reaction which it provoked, has made the deistical moralists almost unanimous in proclaiming the divinity of Nature, and setting up its fancied dictates as an authoritative rule of action. A reference to that supposed standard is the predominant ingredient in the vein of thought and feeling which was opened by Rousseau, and which has infiltrated itself most widely into the modern mind, not excepting that portion of it which calls itself Christian. The doctrines of Christianity have in every age been largely accommodated to the philosophy which happened to be prevalent, and the Christianity of our day has borrowed a considerable part of its colour and flavour from sentimental deism. At the present time it cannot be said that Nature, or any other standard, is applied as it was wont to be, to deduce rules of action with juridical precision, and with an attempt to make its application co-extensive with all human agency. The people of this generation do not commonly apply principles with any such studious exactness, nor own such binding allegiance to any standard, but live in a kind of confusion of many standards; a condition not propitious to the formation of steady moral convictions, but convenient enough to those whose moral opinions sit lightly on them, since it gives them a much wider range of arguments for defending the doctrine of the moment. But though perhaps no one could now be found who, like the institutional writers of former times, adopts the so-called Law of Nature as the foundation of ethics, and endeavours consistently to reason from it, the word and its cognates must still be counted among those which carry great weight in moral argumentation. That any mode of thinking, feeling, or acting, is "according to nature " is usually accepted as a strong argument for its goodness. If it can be said with any plausibility that "nature enjoins" anything, the propriety of obeying the injunction is by most people considered to be made out; and, conversely, the imputation of being contrary to nature is thought to bar the door against any pretension, on the part of the thing so designated, to be tolerated or excused; and the word "unnatural " has not ceased to be one of the most vituperative epithets in the language. Those who deal in these expressions may avoid making themselves responsible for any fundamental theorem respecting the standard of moral obligation, but they do not the less imply such a theorem, and one which must be the same in substance with that on which the more logical thinkers of a more laborious age grounded their systematic treatises on Natural Law.
Is it necessary to recognise in these forms of speech another distinct meaning of the word "nature"? Or can they be connected, by any rational bond of union, with either of the two meanings already treated of? At first it may seem that we have no option but to admit another ambiguity in the term. All inquiries are either into what is or into what ought to be: science and history belonging to the first division; art, morals, and politics to the second. But the two senses of the word "nature" first pointed out agree in referring only to what is. In the first meaning, Nature is a collective name for everything which is. In the second, it is a name for everything which is of itself, without voluntary human intervention. But the employment of the word "nature" as a term of ethics seems to disclose a third meaning, in which Nature does not stand for what is, but for what ought to be, or for the rule or standard of what ought to be. A little consideration, however, will show that this is not a case of ambiguity; there is not here a third sense of the word. Those who set up Nature as a standard of action do not intend a merely verbal proposition; they do not mean that the standard, whatever it be should be called Nature; they think they are giving some information as to what the standard of action really is. Those who say that we ought to act according to Nature do not mean the mere identical proposition that we ought to do what we ought to do. They think that the word "nature" affords some external criterion of what we should do and if they lay down as a rule for what ought to be, a word which in its proper signification denotes what is, they do so because they have a notion, either clearly or confusedly, that what is constitutes the rule and standard of what ought to be.