Origins
The Proto-Indo-European root *men- encodes one of the most fundamentally human concepts: the act of thinking. From this single syllable, spoken millennia before writing, descend words for mind, memory, madness, warning, spiritual practice, and even currency. Its reflexes are found in every major branch of the Indo-European family, with a semantic range that illuminates how ancient speakers understood the relationship between thought, memory, and altered mental states.
The root is reconstructed as *men- with the basic meaning 'to think, to direct one's mind toward something.' It participates in the standard PIE ablaut system: the full-grade *men- appears in formations meaning 'mind' and 'thinking'; the o-grade *mon- produced words meaning 'to remind, to warn' (since warning is causing someone to think); and the zero-grade *mn̥- generated words for memory (the residue of past thought). Each ablaut grade created its own extensive word family.
In Sanskrit, the root is preserved with exceptional transparency. The neuter noun mánas means 'mind, spirit, thought' and is one of the key philosophical terms of the Vedic and later Hindu traditions. The compound mántra (literally 'instrument of thought,' from *men- + the instrumental suffix *-trom) designated a sacred utterance or formula, a word that has been borrowed into English as 'mantra.' The verb mányate (to think, to believe) and the adjective mányu (spirited, passionate) complete the Sanskrit picture.
Proto-Indo-European Roots
In Ancient Greek, the root took several important paths. The noun μένος (ménos) meant 'spirit, force, vital energy' — the mental fire that drives a warrior in Homer. The noun μνήμη (mnḗmē, memory) produced English 'mnemonic' (a memory device) through its adjective μνημονικός (mnēmonikós). The negative prefix a- yielded ἀμνησία (amnēsía, forgetfulness), giving English 'amnesia.' The related ἀμνηστία (amnēstía, a deliberate forgetting of offenses) became 'amnesty.' Perhaps most strikingly, μανία (manía, madness, frenzy) also descends from *men- — the ancient Greeks understood madness not as the absence of mind but as its derangement, mind in excessive or disordered activity. 'Mania,' 'maniac,' and 'manic' all enter English from this Greek derivative. The mythological figure of the Muse (Greek Μοῦσα, Moûsa, possibly from *mon-t-ya, 'she who causes thinking') may also connect to this root, though that etymology is debated.
In Latin, *men- produced the noun mēns (genitive mentis, 'mind'), the direct source of English 'mental,' 'mentality,' and 'mention' (originally 'a calling to mind'). The Latin verb meminī (I remember, a perfect with present meaning) and the noun memoria (memory) generated the English family of 'memory,' 'memoir,' 'memorial,' 'memorize,' 'remember,' 'commemorate,' and 'immemorial.' The o-grade form *mon- yielded Latin monēre (to warn, to remind), producing 'monitor' (one who warns), 'monument' (something that reminds), 'admonish,' 'premonition,' and 'summon.' The word 'monster' also derives from monēre through the Latin monstrum (a divine warning, a portent) — monsters were originally signs from the gods, things that caused people to think.
The connection to 'money' deserves special attention. The Roman mint was located in the temple of Juno Monēta on the Capitoline Hill. 'Monēta' was an epithet of Juno meaning 'the Warner' or 'the Adviser,' from monēre. Because coins were struck in her temple, the epithet 'Monēta' transferred to the coinage itself, eventually producing Old French 'moneie' and English 'money,' 'monetary,' and 'mint.' Thus the word for currency in the world's dominant economic language traces back, through a chain of temple architecture and divine epithets, to the PIE concept of thinking.
Later Development
In the Germanic branch, PIE *men- underwent Grimm's Law but retained its core semantics. Proto-Germanic *mundiz (mind, memory) and *munaną (to think, to remember) produced Old English 'gemynd' (memory, mind) and modern 'mind.' Old Norse 'minne' (memory, love — since to love is to keep in mind) gave German its 'Minne' (courtly love in medieval poetry) and 'Minnesinger' (love-poet). The verb 'mean' (Old English 'mǣnan,' to intend, to have in mind) also connects to this root.
The Slavic branch shows the root in Old Church Slavonic 'mĭnĕti' (to think) and Russian 'память' (pamyat', memory, with a prefix). Lithuanian 'mintis' (thought) and 'minti' (to remember) confirm the Baltic evidence. The Celtic branch preserves it in Old Irish 'do-moiniur' (I think, I believe).
The semantic coherence across six thousand years is remarkable: thinking, remembering, warning, going mad, reciting sacred formulas, and (by a theological accident of Roman temple geography) making money — all spring from a single root that meant, simply, to direct the mind.