A follower of Saint Augustine or his religious and philosophical teachings; can also be an adjective describing those teachings.
From Saint Augustine (Latin 'Augustinus'), the influential Christian theologian and philosopher of the 4th-5th centuries. The '-ian' suffix denotes followers or adherents.
Augustine basically invented the concept of psychological introspection in his 'Confessions'—he's arguably the first person to write about his own mental life like a modern person, making him startlingly relevant 1,600 years later.
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