In Leonardo’s opinion, it was not sufficient for the artist to represent the appearance of things. He should also convey how they moved and functioned as living beings.

Both the dog and the fish in Tobias and the Angel are thought to be the work of the young Leonardo, and as such are evidence of his approach to painting. The dog’s coat has been rendered with great painterly freedom. The fluffy texture of the coat and the dog’s jaunty pose convey a wonderful sense of movement and vitality. The raised forelock and the turn of its head mimics the pose of the Archangel Raphael perfectly. Both appear to stride forward in unison.

The iridescent quality of the fish’s scales and their reflective quality are wonderfully captured. The wriggling body and open-mouth of the fish seem to indicate the life that has barely departed from its body. In a novel concern for naturalistic, narrative detail, Leonardo has also taken care to portray it as bloody to indicate that Tobias has already removed the gall to use as a remedy for restoring his father’s sight.

In Leonardo's words
If you scorn painting, which is the sole imitator of all the manifest works of nature, you will certainly be scorning a subtle invention, which with philosophical and subtle speculation considers all manner of forms; sea, land, trees, animals, grasses, flowers…truly this is a science, the legitimate daughter of nature…