One of the four temperaments, she holds the tool of geometry, yet is surrounded by chaos. She thinks, but cannot act, while the infant scrawling on the slate, who symbolizes Practical Knowledge, can act but not think. This is then, the melancholia of an artist. He cannot achieve perfect beauty, which is known only to God, because he cannot extend his thinking beyond the limitations of space and the physical world.
from The History of Art by H.W. Jansen