idea

(chiefly in the plural) blue devils

crux (e.g. of an argument), the central and defining point of a line of reasoning, its thematic heart

(psychology) idée fixe

received, or accepted, idea; conventional wisdom / catchphrase; platitude

idée mère

to have something in mind, to have an ulterior motive, to be up to something

to be single-minded, to have a one-track mind; to be strong-minded, to know what one wants; to think logically and consistently

(transitive with à) to set someone straight, to provide someone with an attitude adjustment, to sort someone's ideas out for them, to talk sense into someone, to get someone back on the right track / (reflexive) to get one's head straight, to clear one's head, to get one's thoughts in order

(Platonic philosophy) world of ideas, world of forms (higher ideal world of which tangible reality is only a shadow image)