Although painting is essentially a spatial art, it includes a temporal element, the "specious present," the single moment of action or of motion. The lines are not dead and static, but alive; they progress and vibrate; by their means a smile, the rippling of a stream, the gesture of surprise, the movement of a dance, may be depicted. Successive moments, the different phases of an action or movement, cannot, however, be represented. Strict unities of space and time should be observed in painting. Only contiguous parts of space and only one moment of time should be represented inside a single frame. Both these unities were violated in old religious paintings where sometimes the Nativity, Flight into Egypt, Crucifixion, and Resurrection were all portrayed on one canvas. The space of painting is no abstract aspect of things such as the geometer elaborates. To be in a common space with other things, implies, for the pictorial intuition of the world, to be played upon by the same light and to be enveloped in the same atmosphere. Space, light, and air constitute the milieu in which everything lives and moves and has its being in painting. To every difference in the arrangement and foreshortening of objects, to every variation in their lights and shadows and aerial quality, the sensitive soul responds. The close proximity of objects in a tiny room has an effect upon feeling very different from their wide distribution over a broad space. An equal difference depends upon whether light is concentrated upon objects or evenly distributed over them; upon whether it is bright or dim; upon whether they are near and clear in a thin air or far and hazy in a thick and heavy cloud. The masters of light and air, Rembrandt, Claude, Turner, evoke myriad moods through these subtle influences. A long development and the following of many false paths was necessary before painting discovered its true function as an expression of the elements, the once hard outlines of things softening in their enveloping embrace.