I got confused with the author’s definition for the word ‘idea’, and decided to check the etymology. I found it quite fascinating!
late 14c., "archetype, concept of a thing in the mind of God,"
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Meaning "mental image or picture" is from 1610s (the Greek word for it was ennoia, originally "act of thinking"), as is the sense "concept of something to be done; concept of what ought to be, differing from what is observed." Sense of "result of thinking" first recorded 1640s.
late 14c., "archetype, concept of a thing in the mind of God,"
<…>
Meaning "mental image or picture" is from 1610s (the Greek word for it was ennoia, originally "act of thinking"), as is the sense "concept of something to be done; concept of what ought to be, differing from what is observed." Sense of "result of thinking" first recorded 1640s.
Source: https://www.etymonline.com/word/idea