Perhaps it’s like this:
aesthetics : canon : art :: rhetoric : grammar : language
Both pictorial art and language are “free creative” acts, that is, each of them forms and shapes its products (images, words) completely separately from the things pointed at. Over time and within a relatively contiguous community of recipients, norms of how these forms should look or sound arise and are endorsed and retained—canons, standards, conventions, grammars, preferred pronunciations, and ultimately the cultural phenomenon of taste.
Consider how often, and how unnoticed, it is that certain constructions are almost entirely conventional, not truly imitative or “representative,” yet they do not arrest our attention. Outlines themselves are an invisible convention; hatch marks for shading are sometimes an invisible convention. In language (I’ll use English, which I’m most familiar with), structural words (prepositions, conjunctions, etc.) tend to remain invisible until, through repetition, odd locution, or misuse, the reader or listener becomes aware of them.
Rules and guidelines eventually develop to describe how images or language work, why certain forms or presentations can appear to be defective and others quite extraordinary. I suspect the rules were developed as teaching aids to instruct the student how to work efficiently and what to avoid, as practical lore and folk wisdom based on previous success or failure. That’s how the warning against splitting infinitives or ending a sentence with a preposition takes root as hard and fast prohibitions. They were introduced as guide for students, who observed the injunctions with deep reverence, but eventually the wise advice became linguistic fetishes enforced with the power of taboo. Curiously, mastery of technical details, and especially prescriptive rules, has become more highly regarded than true expressive excellence: a cursory review of many academic and policy texts will demonstrate that fact.
We are at the 25,000th year of a long history of teaching and refining techniques—and absorbing new modes and practices from elsewhere—about making and using art and language in society, and the guidelines have become very detailed, extensive, and complicated. Knowledge of them has taken on the trappings of esoteric learning, and adepts are honored publicly.
I’ve often run into the situation that a non-artist really likes one of my paintings that I think is poor because of this and that—blemishes or clumsiness or poor technique or other things I can easily see but that the other person just isn’t attuned to. The other person isn’t schooled in the conventions, and thus is less aware of departures from a norm, from those guidelines that form part of the foundations of taste and aesthetics.