Epicureanism (Google definition)
A Google definition of Epicureanism as an anti-deterministic restrained-hedonism god-free happiness-aiming philosophy, based on the logic that all "things" can be explained on the principle of atoms moving, bonding, forming structures therein, and debonding, in a void, eternally.
In isms, Epicureanism is a physics-based ethical philosophy, developed by Greek philosopher Epicurus (c.307BC), as expounded on by his student Lucretius (55BC), according to which “pleasure”, i.e. “peace and harmony of mind and body” (Close, 1977), is the only good and the end of all morality, thought to be achieved through an actuated existence of simplicity, prudence, honor, and justice, according to which “peace, quite, and contentment” (Close, 1977) were, supposedly, the idea state; it was the second most-popular philosophy below stoicism, in its period. [1]

Jefferson | Synopsis
In circa 1799, Thomas Jefferson, who owned at least five Latin editions of Lucretius’ 55BC On the Nature of Things, penned per his own personal use the following “Syllabus of the doctrines of Epicurus”, which in 1819 attached as a note in a letter to fellow Epicurean William Short: [2]

Syllabus of the Doctrines of Epicurus

Physical: The universe eternal.
Its parts, great and small, interchangeable.
Matter and void alone.
Motion inherent in matter which is weighty and declining.
Eternal circulation of the elements of bodies.
Gods: an order of beings next superior to man, enjoying in their sphere, their own felicities; but not meddling with the concerns of the scale of beings below them.
Moral: Happiness the aim of life.
Virtue: the foundation of happiness.
Utility the test of virtue.
Pleasure: active and In-do-lent.
In-do-lence – is the absence of pain, the true felicity.
Active – consists in agreeable motion; it is not happiness, but the means to produce it.

Thus the absence of hunger is an article of felicity; eating the means to obtain it. The summum Annum [default greatest, highest, or most elevated] is to be not pained in body, nor troubled in mind. i.e. In-do-lence of body, tranquility of mind. To procure tranquility of mind we must avoid desire and fear, the two principal diseases of the mind. Man is a free agent. Virtue consists in 1. Prudence. 2. Temperance. 3. Fortitude. 4. Justice. To which are opposed, 1. Folly. 2. Desire. 3. Fear. 4. Deceit.

In 1820, Jefferson had come to characterize himself, in discourse with others, in sum, as an "Epicurean materialist".

Copley | Synopsis
In 1977, Frank Copley, in his “Introduction” to On the Nature of Things, gave the following synopsis: [3]

Epicureanism was a philosophy that brought peace and quiet rather than inspiration and exhilaration; based on a theory of the exclusive validity of sense perception and on an ethical doctrine that ‘pleasure was the criterion of good’, it lent itself not only to a dull and flat dialectic but also to gross misinterpretation. Although, in the Hellenistic period and later in the Roman, it was second of the two great philosophies (the first was stoicism), yet it trailed far behind its rival and gained relatively few adherents, these chiefly among poets, philosophers, and others of a contemplative rather than an active disposition.

Starting with the question ‘how do we get our knowledge?’, Epicurus concludes that we get it through the sense; sensation, he declared, is the only reliable source of knowledge, and consequently the only source of truth. From this starting point, the philosophy proceeds with nearly relentless logic to explain ever phenomenon—and the correct word is ‘every’—of which the ancient world was aware.”

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Misinterpretation | Food & Drink
Of note, the term “Epicurean”, in modern times, is sometimes mistakenly presented or parlayed as a synonym for some someone with “an appreciation for fine food and drink” (Ѻ), namely someone whose existence revolves around being a drunk glutton, or something to this effect; which does not seem to be representative of the original “physics-based philosophy of ethics”.

The following are related quotes:

Epicureanism held that pleasure is the goal of well-being and that neither god nor providence directs human affairs.”
— Epiphanius (378), The Panarion [4]

“The Epicureans were neither determinists nor atheists [see: functional atheist], although they were commonly accused of being both.”
Frank Copley (1977), “Introduction” to On the Nature of Things (pg. xv)

See also
Epicurean swerve

1. (a) Epicureanism – Merriam-Webster Collegiate Dictionary, 2000.
(b) Copley, Frank O. (1977). “Introduction”, in: Lucretius: On the Nature of Things (§1:vii-xx, pg. vii). W.W. Norton, 2011.
2. Jefferson, Thomas. (1819). “Letter to William Short” (Ѻ), Oct 31.
3. Copley, Frank O. (1977). “Introduction”, in: Lucretius: On the Nature of Things (§1:vii-xx, pg. vii). W.W. Norton, 2011.
4. (a) Epiphanius. (378). The Panarion of Epiphanius of Salamis, Volume One (pg. 10). Brill, 1987.
(b) Stephens, Mitchell. (2014). Imagine There’s No Heaven: How Atheism Helped Shape the Modern World (pg. 47). St. Martin's Press.

External links
Epicureanism – Wikipedia.
Nature of Things – NewEpicurean.com.

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