In M. C. Escher's drawing Three Worlds, water acts as the medium for an encounter between phenomena which otherwise could not meet. Images of tree silhouettes are shown as a reflection, existing in conceptual terms at the same distance below the surface as the real branches above it, though neither the real tree nor its reflection is truly in contact with the water. Dead leaves float on the water: on it but not of it, since only their undersides touch the surface. Under the water, or rather in it, is the hazy vision of a swimming fish, the only physical object represented which is truly in its element. We cannot ‘see’ the water since the only framing device is the physical edge of the picture, but we understand it thus from the manner in which the three worlds interrelate through it.

Somehow, aspects of this drawing put one in mind of that part of Bartók's activity concerned with the forms and structures of Western art music. These act in the same way as Escher's water, in that they form the interface between ideas and phenomena from worlds as different as the fallen leaves, the silhouette and the fish. Some belong themselves firmly within the Western art tradition, most notably that of thematic process: the unfolding of motifs, their transformation through rhythmic and decorative variation, and their subjection to different musical techniques such as ostinato or polyphonic treatment. Other ideas – those we associate with the concept of ‘nature’ music – are designed to evoke atmospheres or, as stylized noises, represent the actual sonorities of birdsong or night creatures.