This famous Latin phrase is first attested in a comedy by Plautus (250-184 BC). There, a character says:
Lupus est homo homini, non homo, non quom qualis sit novit
Wolf is the man for man, not man, when he does not know who is the other.
Erasmus included this phrase in his collection of adages, but his celebrity is due to the modern British philosopher Thomas Hobbes (1588-1679). Hobbes is known today mainly for his groundbreaking developments in the field of political theory (especially in his work Leviathan, whose cover is here reproduced), but he was also a philologist and eminent scholar of classical antiquity, as is evidenced by his -still today frequently reprinted- English translation of Thucydides.
Having experienced the atrocities and cruelties of the English Civil War, one of Hobbes’ central aims was the discovery of the rational principles that could help building a stable political regime. The state of nature (i.e., the absence of government) was considered by Hobbes as a synonym for a permanent state of war of all against all, in which the achievements of civilized life would be impossible. Only a government with concentrated power can, in his conception, avoid that conflict. The Latin phrase homo homini lupus illustrates this idea with great force. If a greater authority does not impose restrictions on their brutal instincts, men behave like beasts ready to fall on each other.
This pessimistic view of human nature has been criticized from every position imaginable, perhaps rightly so. History, however, shows beyond doubt that man is the most formidable predator of its peers.