Static two-dimensional pictorial art - traditional drawing and painting - is the only art where the whole can be perceived in a single, simultaneous act of perception. With literature the mind scans along some point within the whole, as with music, film, theatre, ballet, etc. The act of perception with those forms is time-bound or related. They could be likened to streams progressing from A, the start, to B, the finish, and both the piece and one's mind are progressing together along the journey. Perhaps one can form some kind of inchoate sense of the total form after the event, a bird's eye view of the whole, but this is obviously not a direct act of perception. With these time forms all kinds of digressions are possible to the artist, harmful or not to the unity of the whole. but with the traditional pictorial forms, since the whole can be perceived simultaneously a greater rigidity is required, there is more absolute necessity for unity. Just to add that this simultaneity of perception of the whole is no longer there when the viewer focuses in on certain details and, because of the nature of perception, attention is now centred on aspects of the whole. In the creative process this focusing on details is where the artist may lose himself in 'digressions' - at the more obvious levels, say very intricately painting an eye but not standing back from his work to see that that eye is not positioned correctly.
That the static visual forms may be perceived directly as a whole is not meant as a value judgement - it is simply the way things are, though there is certainly an aesthetically satisfying aspect to this. Just to mention that the three-dimensional forms of sculpture do not quite exist in the same simultaneous way as objects of perception as naturally if one is looking at the front of a piece one cannot simultaneously be looking at the back of it.