In the 15th century Italy's art production encountered a number of humanist influences in the face of the traditional art of the Catholic Church.
On a scientific front, Leonardo DaVinci discovered the true forms of the human body through dissection and sketched his findings. Although it was made illegal by the Church dissecting bodies lead to discoveries that helped understand the human body both functionally and aesthetically. Artists could now create more accurate representations of muscles in action and at rest. In an attempt to achieve ultimate realism, artists started asking quiestions about the world they lived in just as much as those in science.
Another big influence on humanism in art was the Medici family which fueled art production with commissions often of a humanist nature. Their interest in classical Greece spurred a revitalization of nudes and the anatomical proportions in art that had been shunned by the Catholic Church for centuries. Paintings like the Birth of Venus might have been destroyed for its portrayal of nudes if not for the protection of the Medici family. While much of the commissioned art of the past was done in the name of God and about God's glory, the Medici seemed be collecting art as proof of their greatness and the greatness of Florence. This idea of human success without the help of God became definative of Italian humanism.