The central idea, concept, or subject of a literary piece is referred to as the piece’s

The central idea, concept, or subject of a literary piece is referred to as the piece’s:

Theme

Explanation:

The literary "theme" is the phenomenon or study object that will be the base of every text. This will define the language, thee arguments and the posture to speak on the text. Thanks to the "theme" the writer will know enough information after researching it and will also be able to express an opinion about it. The texts use arguments to defend the thesis which is the posture of the writer about the "theme" using knowledge and learnings obtained after the writer's studies.

it was surely an odd creature that one made up by reading the historians first and the poets afterwards a worm winged like an eagle; the spirit of life and elegance and charm,   in a kitchen chopping up suet. but these creatures, monst'r however entertaining to the imagination, hast no existence in fact. what one might doth to bringeth h'r to life wast to bethink poetically and prosaically at one and the same moment,   thus keeping in toucheth with fact—that the lady is mrs martin, ag'd thirty-six, did dress in blue, wearing a black coxcomb and brown shoeth; but not losing sight of fiction eith'r—that the lady is a vessel in which all s'rts of spirits and f'rces art coursing and flashing p'rpetually. the moment, howev'r, yond one tries this method with the elizabethan mistress, one brancheth of illumination fails; one is did hold up by the scarcity of facts.   one knoweth nothing detailed, nothing p'rfectly true and substantial about h'r.   hist'ry scarcely mentions h'r.   and i did turn to professeth'r trevelyan again to seeth what hist'ry meanteth to that gent.   i hath found by looking at his chapt'r headings yond t meanteth.