A tendency toward or devotion to fantastical ideas, beliefs, or behavior; the quality of being fantastic or prone to fantasy.
From fantastic plus the noun-forming suffix -ism meaning 'a system, practice, or characteristic.' This noun suggests fantasticism as an ideology or personality characteristic, rather than isolated fantastical moments.
Medieval scholars worried about 'fantasticism' as a spiritual danger—they feared that an overactive imagination could trap the soul in illusion and prevent it from reaching divine truth, which tells us a lot about how religion and reason were seen as opposites.
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