man is at the center. why inside a circle and a square?
squaring the circle is a mathematical impossibility posed as a problem by ancient geometers (see this). what da vinci is proposing is that humanity breaches the apparent contradiction between the two. human is the perfect instantiation, measurement.
renaissance humanism affected the cultural, political, social, and literary landscape of europe. beginning in florence in the last decades of the 14th century, humanism revived the study of science, philosophy, art and poetry of classical antiquity.