Transfiguration Cathedral in Pereslavl-Zalessky is the oldest white stone church of pre-Mongol Kievan Rus' that survived to this day in its original form. The power of its emotional impact on the viewer was noted by the famous American architect of the XX century Luis Kahn, who travelled around cities of the Golden Ring of Russia in 1973. At the end of this trip, he stated that he was most impressed by the Transfiguration Cathedral.
 We explored the emotional potential of this unique creation of ancient Russian masters using a distinctive theoretical and information model based on the laws of visual perception. Determining the amount of visual information contained in dimensions of the structure, we found that relations between these dimensions belong to a specific proportional system or sequence of numbers.
 We have proposed a mechanism for measuring the strength of the proportional relationship of dimensions that could belong to one structural level or several at once.
 The method of identifying the proportional correlation and strength of proportional relationships is based on determining the information modularity of relations and the values of information modules in the information content of individual relations.
 During the process of information analysis of the Transfiguration Cathedral, it was found that the vast majority of the ratios belong to the two proportional systems: the golden ratio, with a base of 1,618 and the proportional sequence (progression) with a base of 1,477.
 According to the findings of the Russian neurophysiologist P.V. Simonov, a member of the Russian Academy of Sciences, the exact numbers were found in relations between the extreme characteristics of the encephalogram of the human brain. These ratios of frequencies, according to Simonov, correspond with the human emotional states of "aesthetic perception" and "creative act". 
 It is a well-known fact that Leonardo da Vinci and his associate Luca Pacioli developed the golden ratio theory in the early XVI century. This makes us wonder how the ratio of 1,618 and its derivatives came to be used in Kievan Rus in the XII century?
 The construction of temples in Kievan Rus was based mainly on the "time will tell" principle. Keeping that in mind, we suggested that the perfection of the proportional construction of the Transfiguration Cathedral was achieved because of the architects' sharp sensitivity to the visual perception of proportions, supported by various sacred religious practices.
 The presence of the ratio of "aesthetic perception" and the ratio of "creative act" in the proportional structure of the Transfiguration Cathedral explains the strength of its emotional effect on the famous connoisseur of beauty Louis Kahn.