Symmetry is widely believed by traditional aesthetic theorists to be central to the notion of beauty. Archaic systems of architecture and composition such as the Golden Mean and the ratio of 1 to 1.618 formalized ideas of symmetry and balance and applied them to everything from plants to the human body to art and architecture. Symmetry is characterized by balance and evenness. For example, most animals exhibit symmetry in their bodies: a face has an eye on each side, paws, feet or hands are identical on both sides and the mass of a body is evenly distributed.
Composition is a larger concept within which symmetry is embedded. The composition of a landscape, work of art or piece of music involves the placement of its dominant features within the whole and how all of the parts relate to each other. When these relationships are discordant or in conflict, the result is perceived as ugliness. When they are in harmony and exhibit sensible or well-proportioned progressions, the result is perceived as beauty. The dominance of composition within the world of art began to be challenged by modernism in the early 20th century, particularly by "anti-art" movements such as Dada and surrealism.
Postmodern theorists such as Theodor Adorno and Jacques Derrida have been instrumental in deconstructing ideas of objective systems of aesthetics. Postmodernism promotes ideas of radical subjectivity and "slippage," both of which present existence as having no objective meaning and a constantly changing form. This theory doesn't object to the idea that beauty exists but claims that it is created, as Shakespeare noted long ago, "in the eye of the beholder," meaning that beauty is a form of perception and not a reality that exists outside of the mind. The same object may be thought of as beautiful by one person and as ugly by another, implying that these qualities are judgments and not inherent traits.
The field of aesthetics is closely related to art. Although much modern and postmodern art is either indifferent to beauty or actively in opposition to it, prior to these movements beauty and aesthetics were central to the theory and creation of art. Traditional art attempted to express historical tales, religious devotion and moral imperatives while adhering to parameters of beauty that would make this content more accessible to the viewer or listener.