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Why did Stoicism teach that all human beings share a capacity for reason?

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Seneca A 3rd century marble bust of Seneca, after an original bust of the 1st century; in the Staatliche Museen zu Berlin, Germany

Stoicism taught that all human beings share a capacity for reason because they viewed reason as a fundamental, universal element that pervades the world as a divine logos or "divine fire".123

Because this rational capacity is inherent in all people, Stoics argued that humanity is united by a common nature, transcending local affiliations, social status, or birth. This belief led to several key ethical principles:

Universal Equality: Since all individuals possess the same capacity for reason, they are fundamentally equal, a concept that challenged traditional hierarchies and supported the idea of a "universal brotherhood".13
Cosmopolitanism: Stoics viewed the world as a single, unified city. They believed that a wise person should identify as a "citizen of the world" rather than being defined by their specific state or origin.123
Natural Law: The shared capacity for reason allows all people to appreciate a universal moral law, providing a foundation for justice that exists independently of local customs or laws.13

By identifying with this shared reason, individuals can pursue virtue and wisdom, which are under their own control and protected from the unpredictable nature of external circumstances.1