Alberti's picture, by contrast, begins not with the world seen, but with a viewer who is actively looking out at object; The picture is not claimed to be part of vision but is indeed the artist's construction, an expression in paint - And so much are we heir to this view of man, or perhaps more particularly so much are art historians heir to this view of artistic representation, that it is hard to see it as a particular modality and not just the way representational art is.
There is probably no artist or writer who meditated as continuously and as deeply on the relationship between seeing, knowing, and picturing the world as did Leonardo da Vinci.
But if the picture takes the place of the eye then the viewer is nowhere. Though fascinated with appearances, Leonardo fears giving himself over to such demands of total absorption and fears the sacrifice of rational human choices that this notion of picturing assumed.
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