Origins
The Latin word 'mēns' — meaning 'mind, intellect, reason, intention' — is the primary channel through which the Proto-Indo-European concept of thinking entered the Latin-derived stratum of the English vocabulary. Its oblique stem ment- is immediately recognizable in 'mental,' 'mention,' 'comment,' 'dementia,' and many other English words, while its deeper root connects it to some of the most surprising corners of the lexicon.
The PIE root *men- meant 'to think' in a broad sense encompassing thought, memory, intention, and mental force. Its reflexes span the Indo-European world. Sanskrit मनस् (manas, 'mind, spirit') is the central term for the thinking faculty in Indian philosophy, appearing throughout the Upanishads and the Bhagavad Gita. Greek μένος (ménos) meant 'spirit, force, intention' — in Homer, it is the fierce mental energy that drives warriors into battle. The Greek derivatives include Μνημοσύνη (Mnēmosýnē, 'Memory,' mother of the Muses), μνῆμα (mnēma, 'memorial'), and the suffix -μαντις (-mantis, 'seer, diviner'), as in necromancy. Old English had gemynd ('memory, mind'), which became Modern English 'mind.'
In Latin, mēns was a third-declension feminine noun (stem ment-). It covered the entire field of mental activity: intellect, understanding, reason, thought, opinion, intention, plan, disposition, and courage. Cicero used it as the Latin equivalent of Greek νοῦς (noūs, 'intellect'). The phrase 'mēns sāna in corpore sānō' ('a sound mind in a sound body'), from Juvenal's Satires, has been quoted for two thousand years. In Roman law, mēns became a technical term for criminal intent — the ancestor of the English legal doctrine of 'mēns rea' ('guilty mind'), which holds that an act is not criminal unless committed with a culpable mental state.
Proto-Indo-European Roots
The adjective mentālis ('of the mind'), formed in Medieval Latin, gave English 'mental' (first attested c. 1422). 'Mentality' followed. The verb *mentāre is not Classical but lies behind several compounds. 'Mention' comes from Latin mentiō (stem mentiōn-), meaning 'a calling to mind,' from mēns. 'Comment' comes from commentum ('invention, fabrication,' later 'interpretation'), from com- + *men- ('to think together'), via commentārī ('to ponder, to annotate'). 'Commentary' and 'commentator' follow the same path.
The pathological dimension of mēns gave English 'demented' and 'dementia' — from Latin dēmēns (dē- 'away from' + mēns, 'out of one's mind'). The prefix reverses the mind's proper function: to be demented is to have one's mind taken away. 'Amentia' (a- + mēns) meant congenital absence of mind, as distinct from dementia, which implied the loss of a mind once present.
Through the related Latin verb monēre ('to remind, to warn, to advise' — from *mon-ēre, a causative formation from *men-, literally 'to cause to think'), mēns connects to another large word family. 'Monitor' originally meant 'one who reminds or warns.' 'Admonish' comes from ad- + monēre ('to warn toward'). 'Premonition' is a 'warning beforehand.' 'Monument' comes from monumentum ('a reminder, a memorial'), from monēre — because the original function of a monument was to make people think, to remind them of something. Even 'monster' derives from monstrum ('a portent, an unnatural thing'), from monēre — because in Roman religion, a monster was a divine warning, something sent to remind humans of the gods' displeasure.
Later History
'Demonstrate' (from dē- + mōnstrāre, 'to point out, to show') extends the chain further: to demonstrate is to show something clearly, to make it evident — originally, to make it a portent. 'Summon' comes from sub- + monēre ('to remind privately, to call up').
The suffix -ment, one of the most common in English ('government,' 'argument,' 'development,' 'achievement'), derives from the Latin suffix -mentum, which formed nouns of instrument or result from verbs. While -mentum is formally related to mēns (both from *men-), its productivity in Latin was independent. Still, the connection is historically real: the suffix originally implied the means or result of a mental act.
The extended family of *men- in English thus spans an remarkable range: from 'mind' (Germanic) to 'mental' (Latin), from 'memory' and 'memoir' (through Latin memoria) to 'mnemonic' (through Greek μνήμη), from 'monster' and 'monument' (through Latin monēre) to 'mania' (through Greek μανία, 'madness,' from the same root in its sense of 'mental agitation'). The PIE speakers' word for thinking infiltrated nearly every domain of the English vocabulary.