Following the recent death of David Hockney, I came across a video in which he talked about shadows. His argument was that the emergence of cast shadows in Renaissance painting, after their comparatively limited pictorial significance in medieval painting as well as Chinese and Japanese art, was closely connected to the use of optical devices such as mirrors, lenses and the camera obscura.

I must admit that I haven’t read his book Secret Knowledge, where he develops this idea in detail, so I cannot comment on the merits of his argument. Nor is the question of whether painters used such devices one that particularly concerns me. Painting has always been an extraordinarily difficult undertaking, and artists have relied on all kinds of aids throughout history: sketches, cartoons, pouncing, grids, perspective machines, mirrors, lenses, casts, mannequins, photography and, more recently, digital models. In that respect, the use of an optical aid would hardly be surprising.

Curiously, Pliny’s history of painting begins in much the same spirit. According to his account, the origin of painting lies in the tracing of a lover’s cast shadow before his departure. If the story is to be believed, painting itself begins with an optical aid. Yet what is traced is not the shadow itself, but only its outline. The shadow is transformed into a line.

A contour has a curious property. Rather than modelling a form, it separates one thing from another. In doing so, it simultaneously defines an object and the space around it. The same line belongs equally to both.

Many traditions of painting have been built upon this principle. Objects are first established in their own identity. Only afterwards do they begin to relate to one another through their accumulation, overlap and position within the pictorial space.

Medieval painting often works in this way. So does much Japanese painting and printmaking. Here, silhouette and local colour tend to establish the identity of things before they become part of a larger visual order.

Patterns provide another revealing example. In many Islamic miniatures they are applied evenly across the surface of a garment, largely indifferent to folds, perspective or illumination. Hergé uses checked jackets in much the same way in his Tintin comics. Matisse repeatedly does something similar. The pattern belongs to the object before it belongs to light or perspective.

This does not mean such paintings are flat. Space may be created through overlap, scale, atmospheric colour, graduation or vertical placement. But the relations between objects arise primarily through their arrangement.

A cast shadow introduces a completely different kind of relation. It leaves one object, crosses space and falls across another. It ignores contours, changes colour and grows softer or sharper. Unlike the contour, it belongs fully to neither object nor surrounding. It is produced by their interaction.

Once painting begins to rely on such relations, the autonomy of forms gradually gives way to a world governed by interaction. Objects no longer simply occupy the same space. They share a common illumination. Colour becomes increasingly relational rather than local. A shadow cast by one object may become the description of another. Shared illumination creates a common world.