Short Answer: Lucretius records that "Divine Pleasure [is] the guide of life."
Long Answer:
From Norman DeWitt's "Epicurus and His Philosophy" Chapter 12: The New Hedonism
PLEASURE IDENTIFIED AS THE TELOS
When once the summum bonum fallacy has been detected and the difference clearly discerned between the greatest good, which is life itself, and the end or telos, the next step is to apprehend clearly by what procedure the end or telos is identified as pleasure. The nature of this procedure and of the attitude which determined it was one thing in the time of Cicero and quite another in the time of Epicurus himself. In the space of the two centuries between these two men the study of formal logic had been forced into a dominating position in the curriculum through the aggressive genius of the Stoic Chrysippus, and after his time the incessant needling of Stoic adversaries had shaken the confidence of many Epicureans in the word of their founder.3 The faith of Epicurus himself had pinned itself upon Nature as the norm, not upon Reason. The faith of the Stoic, on the contrary, and of those Epicureans who wavered in their faith, while ostensibly pinned upon Reason, may more correctly be said to have been pinned upon argumentation and disputation.
When Epicurus himself identified pleasure as "the end of Nature" he was setting Reason aside and recognizing Nature as the norm or as furnishing the norm. In this he was merely following a trend of his time. The brilliant Eudoxus, for example, who had preceded him by no great interval, also declared pleasure to be the good and he took his start from the observation that all creatures, whether rational or irrational, pursued it.4 Confirmation for the truth of this observation was found in the behavior of all creatures toward pain. If we may accept as authentic the tradition as reported by Aristotle, it would seem that Eudoxus thought of the pursuit of pleasure as comparable to the instinct of wild creatures to seek their proper food and to avoid the opposite. This demonstrates clearly the incipient tendency to recognize Nature as furnishing the norm.5
Thus the originality of Epicurus did not consist in recognizing Nature as furnishing the norm but in working out this principle to its utmost limit, which he did by setting up his Canon, each item of which, Sensations, Anticipations, and Feelings, was a separate appeal to the authority of Nature.
In identifying pleasure as the end or telos it is both possible and probable that Epicurus was taking up a suggestion of Aristotle, who dropped the hint in this instance that the evidence drawn from the behavior of irrational creatures is superior in value to the evidence drawn from the behavior of rational creatures.6 At any rate the declaration of Epicurus, as reported by Cicero, runs as follows: "Every living creature, the moment it is born, reaches out for pleasure and rejoices in it as the highest good, shrinks from pain as the greatest evil, and, so far as it is able, averts it from itself." 7
In the evaluation of this text the important words are "the moment it is born." By narrowing the field of observation to the newborn creature Epicurus was eliminating all differences between rational and irrational'creatures. In infancy even the creatures that by courtesy we call rational are as yet irrational. By narrowing the field to the newborn Epicurus was also reducing animate life to its minimum value, because at the moment of birth even some of the senses have not yet begun to function. Consequently, as Cicero says in the same context, "since nothing is left of a human being when the senses are eliminated, the question, what is according to Nature or contrary to Nature, is of necessity being judged by Nature herself."
It is doubtful whether any other item of Epicurean invention is the equal of this in logical acumen. Even if weight be allowed to the later objection of the Stoics that the behavior of the infant has its cause in what we now call the instinct of self-preservation, this interpretation would lead to the recognition of life as the greatest good, which was the doctrine of Epicurus, and it would still be left for pleasure and pain to function as the criteria.
Incidentally, this appeal to the evidence afforded by the newly born exercised its effect upon the terminology of Epicurus. The infant, being still in a state of nature, is "not yet perverted." These words afford a hint of the perversion ascribed to the study of rhetoric, dialectic, and mathematics, which a lad was judged lucky to have escaped. As for Nature herself, she speaks through the newly born "undefiled and uncontaminated." Her word is "true philosophy," the vera ratio so often invoked by Lucretius.
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