One always starts work with the subject, no matter how tenuous it is, and one constructs an artificial structure by which one can trap the reality of the subject-matter that one has started from.
The human understanding, when any preposition has been once laid down... forces everything else to add fresh support and confirmation; and although more cogent and abundant instances may exist to the contrary, yet it either does not observe them or it despises them, or it gets rid of and rejects them by some distinction, with violent and injurious prejudice, rather than sacrifice the authority of its first conclusions.
I've had photographs taken for portraits because I very much prefer working from the photographs than from models... I couldn't attempt to do a portrait from photographs of somebody I didn't know.
Natural abilities are like natural plants, that need pruning by study; and studies themselves do give forth directions too much at large, except they be bounded in by experience.