Why did Rudolph Reti argue that the themes in a masterpiece cannot be freely exchanged?

Rudolph Reti argued that themes in a masterpiece cannot be freely exchanged because a musical work is not merely a collection of parts, but a structure that diversifies a fundamental unity.1

In his book The Thematic Process in Music, Reti posited that a masterpiece grows from an "all-embracing idea." Consequently, the contrasting themes within a work are not arbitrary; they are intrinsically linked because each represents a different aspect of a single, underlying thought.1

When themes from one work are substituted into another—even if they match in key, meter, or mood—the result lacks the deep sense of structural integrity and unity present in the original composition. While some critics point to composers like J.S. Bach who borrowed materials, Reti would argue that such borrowing is a highly selective creative act, where specific themes are chosen precisely because they serve the overarching unity of the new work. This perspective challenges listeners and critics to consider the profound connection between a work's individual components and its total form.1