Chance might take the form of something like the decisive moment. Even in landscape and architectural photography there is a decisive moment, perhaps when the light and the clouds are right or when no one is in the scene, or when someone is standing in just the right place to show the scale of the subject, or when the indoor light of a building is balanced by the outdoor light. Portrait photography is heavily dependent on chance expressions. Perhaps only studio still life is the exception, and even then, the inspiration of the photographer is an uncertain element.