Monroe C. Beardsley, Aesthetics: Problems in the Philosophy of Criticism. NY: Harcourt Brace and World, 1958, pp. 95-96—decanter and goblet / Monroe Beardsley
A line that turns upon itself so that its ends are fused, and is therefore endless, defines a figure, or what is often called a form. It may be a continuous, unbroken, line-area— that is, an outline— or a circular row of dots, or a group of sketchy line-areas that fuse into a broken line and make a triangle, a pitcher-shape, or an elephant-shape. Consider, for a moment, a square, each of whose sides is a line-area. It does not matter much whether you call this outline a single line, or say that it consists of four lines, one for each side. If we speak in the latter way, we shall want to say that when two lines—as any two sides—fuse together in such a way that they both become parts of a single figure, then they “go with” each other. Line X goes with line Y when they are parts of the same figure.
To explore this concept a little further, we may consider the multiple relevance of lines—a phenomenon much exploited in modern nonrepresentational painting, but one that also throws light upon principles that have always been important in painting. Suppose we draw a large goblet bulging at the top, and a small decanter bulging at the bottom, in such a way that the same line-area serves as the right-hand side of the goblet and as the left-hand side of the decanter. This line belongs to two distinct figures at the same time, or, in other words, goes with two distinct sets of lines. It binds the two figures in a particularly intimate way, by guiding the eye from any point on the outline of either figure continuously to any point on the outline of the other. It is interesting to distinguish varying degrees of intimacy in the connection between two figures, depending upon the connections of their outlines. (1) The figures may be entirely separated as in Italian primitive painting. (2) There may be a broken line that goes from one to another, as in a painting by Rubens or El Greco, where the line of one figure’s leg or back will be picked up by the arm or side of another figure. (3) The same line may belong to both figures, as in the decanter-goblet example, or in paintings by Ozenfant, Gris, Braque, or Picasso in one period—see his Seated Woman (1927)…. (4) There may be strong lines that cut across both figures, throwing their actual outlines into the background, so that in fact the figures are actually submerged and tend to lose their character as figures. This is the principle of camouflage.
related observation by Gestaltist Fritz Heider