In western art positive space is often the space that is closely tied to the subject matter. In a figure/background work the figure is more important and therefore the positive space and the rest is considered to be less important or the negative space. When the figure is within a context then the significance of that context needs to be considered. If that context is important with respect to the subject matter, then it would be positive space. In this work by Renoir the title By the Water suggests the importance of the water. If the context is not important, then it would be negative space. In Leonard0 da Vinci's Mona Lisa the positive space is Mona Lisa and the negative space simply adds an aesthetic background.
Bear in mind that all cultures do not view positive and negative space in the same way.