What did Oskar Kokoschka mean by calling painting a fourth-dimensional art?

Oskar Kokoschka described painting as a fourth-dimensional art to distinguish his creative process from a purely optical representation of the world. He explained that while the three dimensions of space are perceived through the vision of both eyes, the fourth dimension is a "projection of myself".1

For Kokoschka, this fourth dimension represented the "essential nature of vision," which he viewed as a creative act rather than a passive recording of physical reality. By incorporating this subjective, internal element, he moved beyond the limitations of three-dimensional space to express the emotional and psychological depth of his subjects. This philosophy was central to his work as a leading exponent of Expressionism, where the artist's personal, humanistic engagement with the subject remained the fundamental basis of his art throughout his life.123