Diogenes the Cynic (/daɪˈɒdʒɪniːz/, dy-OJ-in-eez; c. 413/403 – c. 324/321 BC), also known as Diogenes of Sinope, was an ancient Greek philosopher and one of the founders of Cynicism. Renowned for his ascetic lifestyle, biting wit, and radical critiques of social conventions, he became a legendary figure whose life and teachings have been recounted, often through anecdote, in both antiquity and later cultural traditions.

Diogenes was born to a prosperous family in Sinope. His life took a dramatic turn following a scandal involving the debasement of coinage, an event that led to his exile and ultimately his radical rejection of conventional values. Embracing a life of poverty and self-sufficiency, he became famous for his unconventional behaviours that openly challenged societal norms, such as living in a jar or wandering public spaces with a lit lantern in daylight, claiming to be "looking for a man". Diogenes advocated for a return to nature, the renunciation of materialism, and introduced early ideas of cosmopolitanism by proclaiming himself a "citizen of the world". His memorable encounters, including that with Alexander the Great, along with various accounts of his death, have made him a lasting symbol of philosophical defiance to established authorities and artificial values.

Diogenes was born c. 413/403 BC in Sinope, a Milesian colony in Paphlagonia on the Black Sea (modern Sinop, northern Turkey). He was the son of Hicesias, a trapezitēs, that is, a moneychanger authorized to exchange foreign currencies for local money. Nothing is known about his mother. As a child, Diogenes learned to read, write, and quote both epic and tragic verses, while also training in athletics and horsemanship. This background reflects his privileged upbringing, as private education was available only to wealthy families. In his father's footsteps, he held the position of epimelētēs, a magistrate whose duties varied by city, though the specifics of his role remain unknown.

In an episode later recast as a metaphor for his philosophy, he and his father were accused of "debasing the currency". Ancient sources disagree on who was responsible: some blame Hicesias, others Diogenes, while a further tradition asserts that Diogenes committed the fraud and fled, whereas his father, who oversaw the treasury, was arrested and died in prison. Modern scholarship remains divided. Some regard this episode as a fiction inspired by Diogenes's writings and symbolic of his aim to "revalue the currency" (paracharattein to nomisma), meaning to challenge conventional values. Others argue for a historical core, citing debased Sinopean coins from 350–340 BC and later issues bearing the name Hikesio. Diogenes himself admitted his guilt in his lost treatise Pordalos, and his father's position as trapezitēs would have made the crime feasible.

A related legend recounts that, seeking guidance from Apollo's oracle at Delphi (or Delos) on how to gain renown, Diogenes was told to "adulterate the currency". Misinterpreting the phrase, he took it literally without realising its figurative sense. Another version places the oracle after his exile, explicitly tying the command to his philosophical mission. This story is likely apocryphal, perhaps a parody of Socrates's Delphic oracle, and inconsistent with Diogenes's scepticism toward religion. He himself claimed to have embraced philosophy only later in Athens. Jean-Manuel Roubineau suggests that the legend may have been devised by later authors to downplay his role in any actual fraud.

The exact date of Diogenes's departure from Sinope is uncertain. It is also unclear whether he was banished or exiled, or if he fled out of fear of the consequences. While ancient tradition holds that Antisthenes (who died c. 366 BC) introduced him to Socratic thought, some modern scholars have questioned the timeline, arguing that Diogenes may have learned about Antisthenes's philosophy only from his writings. Susan H. Prince suggests that he may have arrived in Athens sometime between the 360s and 345 BC, whereas H. Bannert contends that his arrival occurred in the years 370–365. To reconcile those views, some have proposed that he had visited Athens and Antisthenes before his exile, and returned to Sinope.

Diogenes's exile marked a turning point, and a moment of profound spiritual conversion. In his time, being separated from one's homeland, and thus denied the honour of being buried with one's ancestors, was seen as a tragic fate. Diogenes rejected this sentimental attachment, embracing exile as the ultimate detachment from worldly ties. Plutarch notes that the hardships of exile transformed him into a philosopher. In Cynic thought, noble exiles like Odysseus and especially Heracles, about whom Diogenes wrote tragedies, served as models of exemplary behaviour. One anecdote also claims that a chance encounter with a mouse revealed to him the value of a simple life, since the rodent is capable of adapting itself to any circumstance.

Diogenes reportedly owned a Phrygian slave named Manes. Given Diogenes's poverty after fleeing Sinope, it is more likely that Manes was part of his early life rather than a slave bought in Athens. When the slave escaped, Diogenes dismissed his ill fortune by saying, "If Manes can live without Diogenes, why not Diogenes without Manes?". This attitude reflects the Cynic belief that true freedom comes from detaching oneself from possessions and desires, so even owning a slave could be seen as a form of self-enslavement. J. Garcia Gonzalez has argued that this anecdote, like other stories about Diogenes, is symbolic rather than factual, using the names "Manes" and "Diogenes" as generic representations to convey philosophical ideas. Although evading capture, Manes died when wild dogs attacked him as he fled to Delphi.