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Pictorial balance has been used to explain why some paintings evoke feelings of unity and harmony. This study shows how it is calculated and proposes an evolutionary reason for its existence. The concept of pictorial balance as a center of mass effect, i.e. the feeling that somehow a person on the left seems to be balanced by a tree on the right, originated at the beginning of the 20^thcentury. It had never been mentioned in art commentary of the previous 500 years, and yet has become a widely assumed but unsubstantiated explanation of why some paintings evoke feelings of unity and harmony. Despite the lack of historical documentation, the perception of pictorial balance exists. This study shows how the low-level visual system calculates balance, how it evokes feelings of unity and harmony, and its evolutionary importance in visualizing the world of moving organisms. It is done by means of a study of an elusive pictorial effect known to painters, in which these feelings are evoked. In such pictures the image seems to be perceived as a whole without the need to fixate on individual depicted objects. The effect was first described by a 17^thcentury critic Roger de Piles and subsequently by a few renowned artists. The author hypothesized that this response was due to the painting being in a state of perfect balance. Computer modeling found that such a picture must have bilateral luminance symmetry with a lighter lower half by ∼1.07. A study was conducted in which the identical picture seen sequentially in slightly different frames could seem different to observers because of the change in balance created by the frames. The results indicate that the extent to which the pairs were seen as different correlated with two methods of calculating balance. One of them could be understood as a possible primitive visual operating system for identifying other moving organisms as ever-changing vectors of balance. The more balanced the painting, the more it will be seen as a whole object by emphasizing peripheral vision and decreasing the tendency to fixate on what is in the picture, causing the painting to be perceived as unified and harmonious. It is proposed that in the evolution of vision there is a luminance pathway that was later superseded by other pathways using color vision. This explains the percept of balance and other visual phenomena.