Cicero, ius naturale?

De Officiis,
source

{11} IV.

First of all, Nature has endowed every species of living creature with the instinct of self-preservation, of avoiding what seems likely to cause injury to life or limb, and of procuring and providing everything needful for life — food, shelter, and the like. A common property of all creatures is also the reproductive instinct (the purpose of which is the propagation of the species) and also a certain amount of concern for their offspring. But the most marked difference between man and beast is this: the beast, just as far as it is moved by the senses and with very little perception of past or future, adapts itself to that alone which is present at the moment; while man — because he is endowed with reason, by which he comprehends the chain of consequences, perceives the causes of things, understands the relation of cause to effect and of effect to cause, draws analogies, and connects and associates the present and the future — easily surveys the course of his whole life and makes the necessary preparations for its conduct strangely tender love for his offspring. She also prompts men to meet in companies, to form public assemblies and to take part in them themselves; and she further dictates, as a consequence of this, the effort on man's part to provide a store of things that minister to his comforts and wants — and not for himself alone, but for his wife and children and the others whom he holds dear and for whom he ought to provide; and this responsibility also stimulates his courage and makes it stronger for the active duties of life. {13} Above all, the search after truth and its eager pursuit are peculiar to man. And so, when we have leisure from the demands of business cares, we are eager to see, to hear, to learn something new, and we esteem a desire to know the secrets or wonders of creation as indispensable to a happy life. Thus we come to understand that what is true, simple, and genuine appeals most strongly to a man's nature. To this passion for discovering truth there is added a hungering, as it were, for independence, so that a mind well-moulded by Nature is unwilling to be subject to anybody save one who gives rules of conduct or is a teacher of truth or who, for the general good, rules according to justice and law. From this attitude come greatness_of_soul and a sense of superiority to worldly conditions.

{14} And it is no mean manifestation of Nature and Reason that man is the only animal that has a feeling for order, for propriety, for moderation in word and deed. And so no other animal has a sense of beauty, loveliness, harmony in the visible world; and Nature and Reason, extending the analogy of this from the world of sense to the world of spirit, find that beauty, consistency, order are far more to be maintained in thought and deed, and the same Nature and Reason are careful to do nothing in an improper or unmanly fashion, and in every thought and deed to do or think nothing capriciously. It is from these elements that is forged and fashioned that moral goodness which is the subject of this inquiry — something that, even though it be not generally ennobled, is still worthy of all honour;^[2] and by its own nature, we correctly maintain, it merits praise even though it be praised by none.