From time to time I link to Gurney because a lot of his advice on art is more generally applicable to creative endeavor, very much including writing. He makes me think, and I treasure that in other people. This time, though, he quotes Howard Pyle in saying something that runs contrary to my views on writing fiction.
As Howard Pyle said, it is essential that a picture express just one idea. "If in making a picture you introduce two ideas, you weaken it by half—if three, it weakens by compound ratio—if four, the picture will be really too weak to consider at all and the human interest would be entirely lost."
This is almost opposite to how things work in written fiction.
One of the most useful basic definitions of what a story is about is this:
A story is two ideas in conflict.
That's a paraphrase of something either Kris Rusch or Dean Wesley Smith said to me. (I don't know if there's a deeper attribution.)
That definition works at multiple layers. In the seven point plot outline as elucidated by Algis Budrys, we talk about a character in a setting with a problem. A story problem is by definition two ideas (or goals, or people, or situations) in conflict. Without that tension, generally speaking you don't have a story. At least not one that will hold reader interest.
At a higher level, when looking at the plot and structural elements of a story — at any length from flash to doorstop novel — there need to be multiple ideas. The texture and terrain of the narrative will be too flat, to omnidirected, without competing concepts pulling the reader in different directions.
Where Gurney's observation does apply to fiction (and perhaps this is the layer he's operating at with respect to art) is that a story does need an overarching direction, and does need to come to a coherent point. But even then, that direction and resolution comes from the conflict of ideas. For example, where would Charlie Stross' Accelerando be without a veritable kitchen sink admixture of high concept and low cunning?
Multiple ideas multiply the power of fiction. Only to a point, of course, as the value curve of complexity is not asymptotic, but while clarity is a critical vehicle in writing, simplicity can be boring.