4 ideas
Flaubert argues that form and content cannot be separated: a sentence that is badly made expresses a thought badly grasped, so revising the prose is revising the idea. The search for the one exact word (le mot juste) is therefore not decoration but the act of thinking precisely, since each thing has only one right expression.
Flaubert read his sentences aloud, shouting them in what he called the gueuloir, and treated rhythm and sound as proof of a sentence's correctness. A sentence that trips the breath or clangs with repeated sounds is structurally faulty and must be rebuilt, which is why he could spend days on a single page.
The novelist should be in the work like God in the universe, present everywhere and visible nowhere. The author withholds personal opinion and emotional display, letting arranged detail and structure produce meaning, so the reader feels judgment without being told it.
Flaubert treats received ideas and stock phrases (bêtise) as evidence of unexamined thinking: the cliché is a thought nobody actually had, repeated because it is available. Seeing language this way turns every ready-made phrase into something suspect, and makes avoiding it both an aesthetic and a moral discipline, one he pursued in his collecting of banalities for the Dictionary of Received Ideas.