The concept of "beauty" is an illusive and much debated subject. Many philosophers and scholars claimed to have understood and tried to explain what makes something beautiful. Their comparative explanations differ greatly. However, they provide us with precious information as to the prevalent understandings of "beauty" in various epochs of the history of Western civilization.
The Ancient Greeks first introduced the definition of beauty as something that produces pleasant sensations. In those days the keyword to beauty was proportion. Symmetry and harmony, being least likely to cause perceptive uneasiness, were recognized as inherently attractive to the human eye. This concept remained almost undisputed and was accepted with only slight variations in the centuries to come. For example, if we compare the works of the great Renaissance and Impressionist artists, we may find different styles but the striving remains the same: their works induce esthetically pleasant sensations. The 20th century and the horrors of the two great wars introduced a refreshing idea: beautiful needs not be pleasing.
It needs to be fresh, innovative and never-seen-before, even if it entails shocking disproportion and/or disturbing images. Many were the brave theorists who theorized about how tacky and trite the classical concept of beauty was. Cultural studies provided us with another groundbreaking thought: if there is such a thing as universal beauty, how come that the standards of attractiveness differ greatly across cultures? Finally, beauty was allowed, by general consent, to be a matter of individual taste influenced (to an extent) by cultural dictate.
It may have taken us centuries but we have at last acknowledged the old proverb: "Beauty is in the eye of the beholder".