Hellenistic Thought
The Hellenistic World
The great golden age of Athenian philosophy, encompassing Socrates, Plato, and Aristotle only lasted for about a hundred years. In the centuries that followed, changes in the political and cultural climate of the ancient world tended to discourage many varieties of philosophical thinking. The Macedonians under Philip and Alexander founded a Greek empire, which was later conquered by the Romans. Although the general culture of this "Hellenistic" period remained Greek in spirit, political power was vested in a highly centralized state, established and maintained primarily through extensive applications of military force. The (sometime) Athenian tradition of participatory government disappeared as individual citizens were excluded from significantly shaping the social structure of their lives. 
Hellenistic philosophers, therefore, devoted less attention than had Plato and Aristotle to the speculative construction of an ideal state that would facilitate the achievement of a happy life. Instead, the ethical thinkers of this later period focussed upon the life of the individual, independently of the society as a whole, describing in detail the kinds of character and action that might enable a person to live well despite the prevailing political realities. In general, we might say, such philosophers tried to show how we should live when circumstances beyond our control seem to render pointless everything we try to accomplish. The Hellenistic schools of philosophy, then, exhibit less confidence and propose solutions less radical than their Athenian predecessors had in the golden era. 
Epicurus and the Epicureans
The ancient atomists (Leucippus and Democritus) had already worked out a systematic description of the natural world comprising many particular material particles, whose mechanical interactions account for everything that happens. In the Hellenistic period, attention turned to the consequences of such a view for the conduct of human life.
Epicurus and his followers pointed out (in the Principle Doctrines, for example) that since the indestructible atoms that constitute the material world move, swerve, and collide entirely by chance, everything that happens in the universe lies outside the reach of direct human control. (Notice how this position projects Hellenistic political impotence onto the natural world.) Human life is, therefore, essentially passive: all we can do is to experience what goes on, without supposing ourselves capable of changing it. Even so, Epicurus held that this sort of life may be a good one, if the experiences are mostly pleasant ones.
Thus, in the Letter to Menoeceus, Epicurus held that the proper goal of human life is to achieve mental ease {Gk. ataraxia} and freedom from pain. All of our sensual desires are natural and their satisfaction is to be desired, since satiation is always a pleasure but frustrated desire is a mild pain. Material goods are worthwhile only to the extent that possessing them contributes to the achievement of peace. What is more, Epicurus held that we have no reason to complain of the fact that human life must come to an end. Since death results in the annihilation of the personality, he argued, it cannot be experienced and is thus nothing to be feared. Thus, Epicureanism was long ago summarized as the view recommending that we "relax, eat, drink, be merry." (Luke 12:19-20) 
The parody is accurate as far as it goes: Epicurus did suppose that a successful life is one of personal fulfillment and the attainment of happiness within this life. But the philosophical Epicureans were less confident than many of their later imitators about the prospects for achieving very much pleasure in ordinary life. They emphasized instead the mental peace that comes from accepting whatever happens without complaint or struggle.  Pleasure is defined more as the lack of pain than as a positive attritude in itself. Such an attitude is a reasonable response to a natural world and social environment that do not provide for effective individual action and personal security. The Roman philosopher Lucretius defended a similar set of theses, including both atomism in general and an Epicurean devotion to tranquillity in his philosophical poem De Rerum Naturae (On the Nature of Things). 
 Epictetus and the Stoics
A rival school of philosophy in Athens was that of the Stoics. As originally developed by Zeno of Citium and Chrysippus, stoicism offered a comprehensive collection of human knowledge encompassing formal logic, physical study of the natural world, and a thoroughly naturalistic explanation of human nature and conduct. Since each human being is a microcosm of the universe as a whole, they supposed, it is possible to employ the same methods of study to both life and nature equally. 
In the Hellenistic period, Epictetus tersely noted the central features of a life thusly lived according to nature in his Encheiridion (Manual). Once again, the key is to understand how little of what happens is within our control, and stoicism earns its reputation as a stern way of life with recommendations that we accept whatever fate brings us without complaint, concern, or feeling of any kind. Since family, friends, and material goods are all perishable, Epictetus held, we ought never to become attached to them. Instead, we treat everything and everyone we encounter in life as a temporary blessing (or curse), knowing that they will all pass away from us naturally. 
This seems cold and harsh advice indeed, but it works! If, indeed, we form no attachments and care about nothing, then loss will never disturb the tranquillity and peace of our lives. This way of life can be happy even for a slave like Epictetus. But later Roman Stoics like Seneca and Marcus Aurelius made clear in their lives and writings that it has merits even for those who are better off.
 Philosophical Ethics
The Ancient Skeptics
Another school of Hellenistic philosophy illustrates yet again the prevailing lack of confidence that life in this era inspired. The skeptics supposed that the possibility of human knowledge is severely limited in scope and application.
Skepticism began with Pyrrho of Elis, who taught that apart from the sketchy information provided by the senses, we have no genuine knowledge of the nature of things. Unable to achieve certainty about the general structure of the world, human beings should often practice suspension of judgment, which is the only rational response to situations in which they are ignorant. This course naturally results in a nearly total lack of activity, which Pyrrho took to be equivalent to peace of mind. Although he wrote nothing, Pyrrho exerted a powerful influence on succeeding generations through his disciple, Timon of Philius and members of the later Academy. 
Centuries later, Sextus Empiricus wrote a history of skeptical philosophy, the Outlines of Pyrrhonism, and used the Pyrrhonian approach to criticize the pretensions of other schools of thought. He made it clear that the skeptical challenge to traditional theories of knowledge arises from an unusually strict definition of knowledge itself. If we can only be said properly to know what is absolutely certain or beyond doubt, then very little indeed will be known. Although it was widely ignored in his own time, the work of Sextus was instrumental in the modern revival of interest in skeptical philosophy.