Stoicism is a school of Hellenistic philosophy that flourished in Ancient Greece and Ancient Rome. The Stoics believed that the practice of virtue is enough to achieve eudaimonia: a well-lived life. The Stoics identified the path to achieving it with a life spent practicing the four virtues in everyday life—wisdom, courage, temperance or moderation, and justice—as well as living in accordance with nature. It was founded in the ancient Agora of Athens by Zeno of Citium around 300 BCE.

Alongside Aristotle's ethics, the Stoic tradition forms one of the major founding approaches to virtue ethics. The Stoics are especially known for teaching that "virtue is the only good" for human beings, and that external things, such as health, wealth, and pleasure, are not good or bad in themselves (adiaphora) but have value as "material for virtue to act upon". Many Stoics—such as Seneca and Epictetus—emphasized that because "virtue is sufficient for happiness", a sage would be emotionally resilient to misfortune. The Stoics also held that certain destructive emotions resulted from errors of judgment, and they believed people should aim to maintain a will (called prohairesis) that is "in accordance with nature". Because of this, the Stoics thought the best indication of an individual's philosophy was not what a person said but how a person behaved. To live a good life, one had to understand the rules of the natural order since they believed everything was rooted in nature.

Stoicism flourished throughout the Roman and Greek world until the 3rd century CE, and among its adherents was Roman Emperor Marcus Aurelius. It experienced a decline after Christianity became the state religion in the 4th century CE. Since then, it has seen revivals, notably in the Renaissance (Neostoicism) and in the contemporary era (modern Stoicism).

Philosophy does not promise to secure anything external for man, otherwise it would be admitting something that lies beyond its proper subject-matter. For as the material of the carpenter is wood, and that of statuary bronze, so the subject-matter of the art of living is each person's own life.

— Epictetus, Discourses 1.15.2, Robin Hard revised translation

The Stoics provided a unified account of the world, constructed from ideals of logic, monistic physics and naturalistic ethics. Of these, they emphasized ethics as the main focus of human knowledge, though their logical theories were of more interest for later philosophers.

Stoicism teaches the development of self-control and fortitude as a means of overcoming destructive emotions; the philosophy holds that becoming a clear and unbiased thinker allows one to understand the universal reason (logos). Stoicism's primary aspect involves improving the individual's ethical and moral well-being: "Virtue consists in a will that is in agreement with Nature." This principle also applies to the realm of interpersonal relationships; "to be free from anger, envy, and jealousy", and to accept even slaves as "equals of other men, because all men alike are products of nature".

The Stoic ethic espouses a deterministic perspective; in regard to those who lack Stoic virtue, Cleanthes once opined that the wicked man is "like a dog tied to a cart, and compelled to go wherever it goes". A Stoic of virtue, by contrast, would amend his will to suit the world and remain, in the words of Epictetus, "sick and yet happy, in peril and yet happy, dying and yet happy, in exile and happy, in disgrace and happy", thus positing a "completely autonomous" individual will, and at the same time a universe that is "a rigidly deterministic single whole". This viewpoint was later described as "Classical Pantheism" (and was adopted by Dutch philosopher Baruch Spinoza).

Philosopher Julian Baginni has characterized what constitutes one being a Stoic:

"To become a stoic is to endorse the truthfulness of its worldview and accept its prescription for how you ought to live, not just to like how it makes you feel."

Baggini asserts that the endorsement of the truthfulness of the Stoic worldview and the tenets for one's behavior that follow from it, are central to what it means to be a Stoic.

The name Stoicism derives from the Stoa Poikile (Ancient Greek: ἡ ποικίλη στοά), or "painted porch", a colonnade decorated with mythic and historical battle scenes on the north side of the Agora in Athens where Zeno of Citium and his followers gathered to discuss their ideas, near the end of the 4th century BC. Unlike the Epicureans, Zeno chose to teach his philosophy in a public space. Stoicism was originally known as Zenonism. However, this name was soon dropped, likely because the Stoics did not consider their founders to be perfectly wise and to avoid the risk of the philosophy becoming a cult of personality.

Zeno's ideas developed from those of the Cynics (brought to him by Crates of Thebes), whose founding father, Antisthenes, had been a disciple of Socrates. Zeno's most influential successor was Chrysippus, who followed Cleanthes as leader of the school, and was responsible for molding what is now called Stoicism. Stoicism became the foremost popular philosophy among the educated elite in the Hellenistic world and the Roman Empire to the point where, in the words of Gilbert Murray, "nearly all the successors of Alexander [...] professed themselves Stoics". Later Roman Stoics focused on promoting a life in harmony within the universe within which we are active participants.

Scholars usually divide the history of Stoicism into three phases: the Early Stoa, from Zeno's founding to Antipater; the Middle Stoa, including Panaetius and Posidonius; and the Late Stoa, including Musonius Rufus, Seneca, Epictetus, and Marcus Aurelius. No complete works survived from the first two phases of Stoicism. Only Roman texts from the Late Stoa survived.

Philosophy does not promise to secure anything external for man, otherwise it would be admitting something that lies beyond its proper subject-matter. For as the material of the carpenter is wood, and that of statuary bronze, so the subject-matter of the art of living is each person's own life.

— Epictetus, Discourses 1.15.2, Robin Hard revised translation

Of all the schools of ancient philosophy, Stoicism made the greatest claim to being utterly systematic. In the view of the Stoics, philosophy is the practice of virtue, and virtue, the highest form of which is utility, is generally speaking, constructed from ideals of logic, monistic physics, and naturalistic ethics. These three ideals constitute virtue which is necessary for 'living a well reasoned life', seeing as they are all parts of a logos, or philosophical discourse, which includes the mind's rational dialogue with itself. Of them, the Stoics emphasized ethics as the main focus of human knowledge, though their logical theories were of more interest for later philosophers.

Stoicism teaches the development of self-control as a means of overcoming destructive emotions; the philosophy holds that becoming a clear and unbiased thinker allows one to understand the universal reason (logos). Stoicism's primary aspect involves improving the individual's ethical and moral well-being: "Virtue consists in a will that is in agreement with Nature". This principle also applies to the realm of interpersonal relationships; "to be free from anger, envy, and jealousy", and to accept even slaves as "equals of other men, because all men alike are products of nature".

The Stoic ethic espouses a deterministic perspective; in regard to those who lack Stoic virtue, Cleanthes once opined that the wicked man is "like a dog tied to a cart, and compelled to go wherever it goes". A Stoic of virtue, by contrast, would amend his will to suit the world and remain, in the words of Epictetus, "sick and yet happy, in peril and yet happy, dying and yet happy, in exile and happy, in disgrace and happy", thus positing a "completely autonomous" individual will and at the same time a universe that is "a rigidly deterministic single whole". This viewpoint was later described as "Classical Pantheism" (and was adopted by Dutch philosopher Baruch Spinoza).

Diodorus Cronus, who was one of Zeno's teachers, is considered the philosopher who first introduced and developed an approach to logic now known as propositional logic, which is based on statements or propositions, rather than terms, differing greatly from Aristotle's term logic. Later, Chrysippus developed a system that became known as Stoic logic and included a deductive system, Stoic Syllogistic, which was considered a rival to Aristotle's Syllogistic (see Syllogism). New interest in Stoic logic came in the 20th century, when important developments in logic were based on propositional logic. Susanne Bobzien wrote, "The many close similarities between Chrysippus's philosophical logic and that of Gottlob Frege are especially striking".

Bobzien also notes that, "Chrysippus wrote over 300 books on logic, on virtually any topic logic today concerns itself with, including speech act theory, sentence analysis, singular and plural expressions, types of predicates, indexicals, existential propositions, sentential connectives, negations, disjunctions, conditionals, logical consequence, valid argument forms, theory of deduction, propositional logic, modal logic, tense logic, epistemic logic, logic of suppositions, logic of imperatives, ambiguity and logical paradoxes".

The Stoics held that all beings (ὄντα)—though not all things (τινά)—are material. Besides the existing beings they admitted four incorporeals (asomata): time, place, void, and sayable. They were held to be just 'subsisting' while such a status was denied to universals. Thus, they accepted Anaxagoras's idea (as did Aristotle) that if an object is hot, it is because some part of a universal heat body had entered the object. But, unlike Aristotle, they extended the idea to cover all accidents. Thus, if an object is red, it would be because some part of a universal red body had entered the object.

They held that there were four categories:

1. Substance (ὑποκείμενον): The primary matter, formless substance, (ousia) that things are made of
2. Quality (ποιόν): The way matter is organized to form an individual object; in Stoic physics, a physical ingredient (pneuma: air or breath), which informs the matter
3. Somehow disposed (πως ἔχον): Particular characteristics, not present within the object, such as size, shape, action, and posture
4. Somehow disposed in relation to something (πρός τί πως ἔχον): Characteristics related to other phenomena, such as the position of an object within time and space relative to other objects

The Stoics outlined that our own actions, thoughts, and reactions are within our control. The opening paragraph of the Enchiridion states the categories as: "Some things in the world are up to us, while others are not. Up to us are our faculties of judgment, motivation, desire, and aversion. In short, whatever is our own doing." These suggest a space that is up to us or within our power. A simple example of the Stoic categories in use is provided by Jacques Brunschwig:

I am a certain lump of matter, and thereby a substance, an existent something (and thus far that is all); I am a man, and this individual man that I am, and thereby qualified by a common quality and a peculiar one; I am sitting or standing, disposed in a certain way; I am the father of my children, the fellow citizen of my fellow citizens, disposed in a certain way in relation to something else.

The Stoics propounded that knowledge can be attained through the use of reason. Truth can be distinguished from fallacy—even if, in practice, only an approximation can be made. According to the Stoics, the senses constantly receive sensations: pulsations that pass from objects through the senses to the mind, where they leave an impression in the imagination (phantasiai) (an impression arising from the mind was called a phantasma).

The mind has the ability to judge (συγκατάθεσις, synkatathesis)—approve or reject—an impression, enabling it to distinguish a true representation of reality from one that is false. Some impressions can be assented to immediately, but others can achieve only varying degrees of hesitant approval, which can be labeled belief or opinion (doxa). It is only through reason that we gain clear comprehension and conviction (katalepsis). Certain and true knowledge (episteme), achievable by the Stoic sage, can be attained only by verifying the conviction with the expertise of one's peers and the collective judgment of humankind.

According to the Stoics, the Universe is a material reasoning substance (logos), which was divided into two classes: the active and the passive. The passive substance is matter, which "lies sluggish, a substance ready for any use, but sure to remain unemployed if no one sets it in motion". The active substance is an intelligent aether or primordial fire, which acts on the passive matter:

The universe itself is God and the universal outpouring of its soul; it is this same world's guiding principle, operating in mind and reason, together with the common nature of things and the totality that embraces all existence; then the foreordained might and necessity of the future; then fire and the principle of aether; then those elements whose natural state is one of flux and transition, such as water, earth, and air; then the sun, the moon, the stars; and the universal existence in which all things are contained.

— Chrysippus, in Cicero, De Natura Deorum, i. 39

Everything is subject to the laws of Fate, for the Universe acts according to its own nature, and the nature of the passive matter it governs. The souls of humans and animals are emanations from this primordial Fire, and are, likewise, subject to Fate:

Constantly regard the universe as one living being, having one substance and one soul; and observe how all things have reference to one perception, the perception of this one living being; and how all things act with one movement; and how all things are the cooperating causes of all things that exist; observe too the continuous spinning of the thread and the structure of the web.

— Marcus Aurelius, Meditations, iv. 40

Individual souls are perishable by nature, and can be "transmuted and diffused, assuming a fiery nature by being received into the seminal reason ("logos spermatikos") of the Universe". Since right Reason is the foundation of both humanity and the universe.

Stoic theology is a fatalistic and naturalistic pantheism: God is never fully transcendent but always immanent, and identified with Nature. Abrahamic religions personalize God as a world-creating entity, but Stoicism equates God with the totality of the universe; according to Stoic cosmology, which is very similar to the Hindu conception of existence, there is no absolute start to time, as it is considered infinite and cyclic. Similarly, space and the Universe have neither start nor end, rather they are cyclical. The current Universe is a phase in the present cycle, preceded by an infinite number of Universes, doomed to be destroyed ("ekpyrōsis", conflagration) and re-created again, and to be followed by another infinite number of Universes. Stoicism considers all existence as cyclical, the cosmos as eternally self-creating and self-destroying (see also Eternal return).

Stoicism does not posit a beginning or end to the Universe. According to the Stoics, the logos was the active reason or anima mundi pervading and animating the entire Universe. It was conceived as material and is usually identified with God or Nature. The Stoics also referred to the seminal reason ("logos spermatikos"), or the law of generation in the Universe, which was the principle of the active reason working in inanimate matter. Humans, too, each possess a portion of the divine logos, which is the primordial Fire and reason that controls and sustains the Universe.

The foundation of Stoic ethics is that good lies in the state of the soul itself, in wisdom and self-control. One must therefore strive to be free of the passions. For the Stoics, reason meant using logic and understanding the processes of nature—the logos or universal reason, inherent in all things. The Greek word pathos was a wide-ranging term indicating an infliction one suffers. The Stoics used the word to discuss many common emotions such as anger, fear and excessive joy. A passion is a disturbing and misleading force in the mind which occurs because of a failure to reason correctly.

For the Stoic Chrysippus, the passions are evaluative judgements. A person experiencing such an emotion has incorrectly valued an indifferent thing. A fault of judgement, some false notion of good or evil, lies at the root of each passion. Incorrect judgement as to a present good gives rise to delight, while lust is a wrong estimate about the future. Unreal imaginings of evil cause distress about the present, or fear for the future. The ideal Stoic would instead measure things at their real value, and see that the passions are not natural. To be free of the passions is to have a happiness which is self-contained. There would be nothing to fear—for unreason is the only evil; no cause for anger—for others cannot harm you.

The Stoics arranged the passions under four headings: distress, pleasure, fear and lust. One report of the Stoic definitions of these passions appears in the treatise On Passions by Chrysippus (trans. Long & Sedley, pg. 411, modified):

• Distress (lupē): Distress is an irrational contraction, or a fresh opinion that something bad is present, at which people think it right to be depressed.
• Fear (phobos): Fear is an irrational aversion, or avoidance of an expected danger.
• Lust (epithumia): Lust is an irrational desire, or pursuit of an expected good but in reality bad.
• Delight (hēdonē): Delight is an irrational swelling, or a fresh opinion that something good is present, at which people think it right to be elated.

	Present	Future
Good	Delight	Lust
Evil	Distress	Fear

Two of these passions (distress and delight) refer to emotions currently present, and two of these (fear and lust) refer to emotions directed at the future. Thus there are just two states directed at the prospect of good and evil, but subdivided as to whether they are present or future: Numerous subdivisions of the same class were brought under the head of the separate passions:

• Distress: Envy, Rivalry, Jealousy, Compassion, Anxiety, Mourning, Sadness, Troubling, Grief, Lamenting, Depression, Vexation, Despondency.
• Fear: Sluggishness, Shame, Fright, Timidity, Consternation, Pusillanimity, Bewilderment, and Faintheartedness.
• Lust: Anger, Rage, Hatred, Enmity, Wrath, Greed, and Longing.
• Delight: Malice, Rapture, and Ostentation.

The wise person (sophos) is someone who is free from the passions (apatheia). Instead the sage experiences good-feelings (eupatheia) which are clear-headed. These emotional impulses are not excessive, but nor are they diminished emotions. Instead they are the correct rational emotions. The Stoics listed the good-feelings under the headings of joy (chara), wish (boulesis), and caution (eulabeia). Thus if something is present which is a genuine good, then the wise person experiences an uplift in the soul—joy (chara). The Stoics also subdivided the good-feelings:

• Joy: Enjoyment, Cheerfulness, Good spirits
• Wish: Good intent, Goodwill, Welcoming, Cherishing, Love
• Caution: Moral shame, Reverence

The Stoics accepted that suicide was permissible for the wise person in circumstances that might prevent them from living a virtuous life, such as if they fell victim to severe pain or disease, but otherwise suicide would usually be seen as a rejection of one's social duty. For example, Plutarch reports that accepting life under tyranny would have compromised Cato's self-consistency (constantia) as a Stoic and impaired his freedom to make the honorable moral choices.

A distinctive feature of Stoicism is its cosmopolitanism; according to the Stoics, all people are manifestations of the one universal spirit and should live in brotherly love and readily help one another. In the Discourses, Epictetus comments on man's relationship with the world: "Each human being is primarily a citizen of his own commonwealth; but he is also a member of the great city of gods and men, whereof the city political is only a copy." This sentiment echoes that of Diogenes of Sinope, who said, "I am not an Athenian or a Corinthian, but a citizen of the world."

They held that external differences, such as rank and wealth, are of no importance in social relationships. Instead, they advocated the brotherhood of humanity and the natural equality of all human beings. Stoicism became the most influential school of the Greco-Roman world, and produced a number of remarkable writers and personalities, such as Cato the Younger and Epictetus.

In particular, they were noted for their urging of clemency toward slaves. Seneca exhorted, "Kindly remember that he whom you call your slave sprang from the same stock, is smiled upon by the same skies, and on equal terms with yourself breathes, lives, and dies."

Plotinus criticized both Aristotle's Categories and those of the Stoics. His student Porphyry, however, defended Aristotle's scheme. He justified this by arguing that they be interpreted strictly as expressions, rather than as metaphysical realities. The approach can be justified, at least in part, by Aristotle's own words in The Categories. Boethius' acceptance of Porphyry's interpretation led to their being accepted by Scholastic philosophy.

The Fathers of the Church regarded Stoicism as a "pagan philosophy"; nonetheless, early Christian writers employed some of the central philosophical concepts of Stoicism. Examples include the terms "logos", "virtue", "Spirit", and "conscience". But the parallels go well beyond the sharing and borrowing of terminology. Both Stoicism and Christianity assert an inner freedom in the face of the external world, a belief in human kinship with Nature or God, a sense of the innate depravity—or "persistent evil"—of humankind, and the futility and temporary nature of worldly possessions and attachments. Both encourage Ascesis with respect to the passions and inferior emotions, such as lust, and envy, so that the higher possibilities of one's humanity can be awakened and developed. Stoic influence can also be seen in the works of Ambrose of Milan, Marcus Minucius Felix, and Tertullian.

The modern usage is a "person who represses feelings or endures patiently". The Stanford Encyclopedia of Philosophy's entry on Stoicism notes, "the sense of the English adjective 'stoical' is not utterly misleading with regard to its philosophical origins".

The revival of Stoicism in the 20th century can be traced to the publication of Problems in Stoicism by A. A. Long in 1971, and also as part of the late 20th-century surge of interest in virtue ethics. Contemporary Stoicism draws from the late 20th- and early 21st-century spike in publications of scholarly works on ancient Stoicism. Beyond that, the current Stoicist movement traces its roots to the work of Albert Ellis, who developed rational emotive behavior therapy, as well as Aaron T. Beck, who is regarded by many as the father of early versions of cognitive behavioral therapy (CBT).

Stoic philosophy was the original philosophical inspiration for modern cognitive psychotherapy, particularly as mediated by Albert Ellis' Rational-Emotive Behaviour Therapy (REBT), the major precursor of CBT. The original cognitive therapy treatment manual for depression by Aaron T. Beck et al. states, "The philosophical origins of cognitive therapy can be traced back to the Stoic philosophers". A well-known quotation from Enchiridion of Epictetus was taught to most clients during the initial session of traditional REBT by Ellis and his followers: "It's not the events that upset us, but our judgments about the events."

This subsequently became a common element in the socialization phase of many other approaches to CBT. The question of Stoicism's influence on modern psychotherapy, particularly REBT and CBT, was described in detail in The Philosophy of Cognitive–Behavioural Therapy by Donald Robertson. Several early 20th-century psychotherapists were influenced by Stoicism, most notably the "rational persuasion" school founded by the Swiss neurologist and psychotherapist Paul Dubois, who drew heavily on Stoicism in his clinical work and encouraged his clients to study passages from Seneca the Younger as homework assignments.

Similarities of modern Stoicism and third-wave CBT have been suggested as well, and individual reports of its potency in treating depression have been published. There has also been interest in applying the tenets of ancient Stoicism to the human origin story, environmental education, vegetarianism and the modern challenges of sustainable development, material consumption and consumerism.

Seamus Mac Suibhne has described the practices of spiritual exercises as influencing those of reflective practice. Many parallels between Stoic spiritual exercises and modern cognitive behavioral therapy have been identified. According to philosopher Pierre Hadot, philosophy for a Stoic is not just a set of beliefs or ethical claims; it is a way of life involving constant practice and training (or "askēsis"), an active process of constant practice and self-reminder. Epictetus in his Discourses, distinguished between three types of act: judgment, desire, and inclination. which Hadot identifies these three acts with logic, physics and ethics respectively. Hadot writes that in the Meditations, "Each maxim develops either one of these very characteristic topoi [i.e., acts], or two of them or three of them."

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• Baltzly, Dirk. "Stoicism". In Zalta, Edward N. (ed.). Stanford Encyclopedia of Philosophy.
• "Stoicism". Internet Encyclopedia of Philosophy.
• "Stoic Ethics". Internet Encyclopedia of Philosophy.
• "Stoic Philosophy of Mind". Internet Encyclopedia of Philosophy.
• Hicks, Robert Drew (1911). "Stoics" . Encyclopædia Britannica (11th ed.).
• The Stoic Therapy eLibrary Archived 28 February 2021 at the Wayback Machine
• The Stoic Library Archived 25 December 2014 at the Wayback Machine
• Stoic Logic: The Dialectic from Zeno to Chrysippus
• Annotated Bibliography on Ancient Stoic Dialectic
• "A bibliography on Stoicism by the Stoic Foundation". Archived from the original on 1 November 2012. Retrieved 14 September 2012.
• BBC Radio 4's In Our Time programme on Stoicism (requires Flash)
• The Stoic Registry (formerly New Stoa) :Online Stoic Community
• Modern Stoicism (Stoic Week and Stoicon)