Impulse or appetition was the principle in the soul which impelled to action. In an unperverted state it was directed only to things in accordance with nature. The negative form of this principle or the avoidance of things as being contrary to nature, we shall call repulsion.

Notwithstanding the sublime heights to which Stoic morality rose. It was professedly based on self-love, wherein the Stoics were at one with the other schools of thought in the ancient world.

The earliest impulse that appeared in a newly born animal was to protect itself and its own constitution which were conciliated to it by nature. What tended to its survival, it sought; what tended to its destruction, it shunned. Thus self-preservation was the first law of life.

While man was still in the merely animal stage, and before reason was developed in him, the things that were in accordance with his nature were such as health, strength, good bodily condition, soundness of all the senses, beauty, swiftness—in short all the qualities that went to make up richness of physical life and that contributed to the vital harmony. These were called the first things in accordance with nature. Their opposites were all contrary to nature, such as sickness, weakness, mutilation. Under the first things in accordance with nature came also congenial advantages of soul such as quickness of intelligence, natural ability, industry, application, memory, and the like. It was a question whether pleasure was to be included among the number. Some members of the school evidently thought that it might be, but the orthodox opinion was that pleasure was a sort of aftergrowth and that the direct pursuit of it was deleterious to the organism. The after growths of virtue were joy, cheerfulness, and the like. These were the gambolings of the spirit like the frolicsomeness of an animal in the full flush of its vitality or like the blooming of a plant. For one and the same power manifested itself in all ranks of nature, only at each stage on a higher level. To the vegetative powers of the plant the animal added sense and Impulse. It was in accordance therefore with the nature of an animal to obey the Impulses of sense, but to sense and Impulse man superadded reason so that when he became conscious of himself as a rational being, it was in accordance with his nature to let all his Impulses be shaped by this new and master hand. Virtue was therefore pre-eminently in accordance with nature. What then we must now ask is the relation of reason to impulse as conceived by the Stoics? Is reason simply the guiding, and impulse the motive power? Seneca protests against this view, when impulse is identified with passion. One of his grounds for doing so is that reason would be put on a level with passion, if the two were equally necessary for action. But the question is begged by the use of the word 'passion,' which was defined by the Stoics as 'an excessive impulse.' Is it possible then, even on Stoic principles, for reason to work without something different from itself to help it? Or must we say that reason is itself a principle of action? Here Plutarch comes to our aid, who tells us on the authority of Chrysippus in his work on Law that impulse is 'the reason of man commanding him to act,' and similarly that repulsion is 'prohibitive reason.' This renders the Stoic position unmistakable, and we must accomodate our minds to it in spite of its difficulties. Just as we have seen already that reason is not something radically different from sense, so now it appears that reason is not different from impulse, but itself the perfected form of impulse. Whenever impulse is not identical with reason—at least in a rational being—it is not truly impulse, but passion.

The Stoics, it will be observed, were Evolutionists in their psychology. But, like many Evolutionists at the present day, they did not believe in the origin of mind out of matter. In all living things there existed already what they called 'seminal reasons,' which accounted for the intelligence displayed by plants as well as by animals. As there were four cardinal virtues, so there were four primary passions. These were delight, grief, desire and fear. All of them were excited by the presence or the prospect of fancied good or ill. What prompted desire by its prospect caused delight by its presence, and what prompted fear by its prospect caused grief by its presence. Thus two of the primary passions had to do with good and two with evil. All were furies which infested the life of fools, rendering it bitter and grievous to them; and it was the business of philosophy to fight against them. Nor was this strife a hopeless one, since the passions were not grounded in nature, but were due to false opinion. They originated in voluntary judgements, and owed their birth to a lack of mental sobriety. If men wished to live the span of life that was allotted to them in quietness and peace, they must by all means keep clear of the passions.

The four primary passions having been formulated, it became necessary to justify the division by arranging the specific forms of feeling under these four heads. In this task the Stoics displayed a subtlety which is of more interest to the lexicographer than to the student of philosophy. They laid great stress on the derivation of words as affording a clue to their meaning; and, as their etymology was bound by no principles, their ingenuity was free to indulge in the wildest freaks of fancy.

Though all passion stood self-condemned, there were nevertheless certain 'eupathies,' or happy affections, which would be experienced by the ideally good and wise man. These were not perturbations of the soul, but rather 'constancies'; they were not opposed to reason, but were rather part of reason. Though the sage would never be transported with delight, he would still feel an abiding 'joy' in the presence of the true and only good; he would never indeed be agitated by desire, but still he would be animated by 'wish,' for that was directed only to the good; and though he would never feel fear still he would be actuated in danger by a proper caution.

There was therefore something rational corresponding to three out of four primary passions—against delight was to be set joy; against grief there was nothing to be set, for that arose from the presence of ill which would rather never attach to the sage. Grief was the irrational conviction that one ought to afflict oneself where there was no occasion for it. The ideal of the Stoics was the unclouded serenity of Socrates of whom Xanthippe declared that he had always the same face whether on leaving the house In the morning or on returning to it at night.

As the motley crowd of passions followed the banners of their four leaders so specific forms of feeling sanctioned by reason were severally assigned to the three eupathies.

Things were divided by Zeno into good, bad, and indifferent. To good belonged virtue and what partook of virtue; to bad, vice and what partook of vice. All other things were indifferent.