De officiis (ed. 2)

the foregoing: For Panaetius was a Stoic, and the Stoics did not admit that there were any degrees of right or wrong. for we usually consider not only whether an action is morally right or morally wrong, but also, when a choice of two morally right courses is offered, which one is morally better; and likewise, when a choice of two expedients is offered, which one is more expedient. Thus the question which Panaetius thought threefold ought, we find, to be divided into five parts. First, therefore, we must discuss the moral—and that, under two sub-heads; secondly, in the same manner, the expedient; and finally, the cases where they must be weighed against each other. First of all, Nature has endowed every species The essential differences between man and the lower animals. 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 Instinct and Reason. 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.…