what is differing definitions of art?

DIFFERING DEFINITIONS OF ART: FROM PLATO TO THE POSTMODERN
So far, we have considered definitions of art as imitation; significant form; expression; as an activity understood in terms of the metaphor of ‘family resemblance’; or as something which is institutionally
created. We have noted inconclusive claims as to what constitutes beauty and how it might be justified. These approaches provide a theoretical context for understanding some of the differing historical approaches to, and contexts of, artistic production. Although this is a sweeping generalisation, in a Western context the first theory, or variations of it, was prevalent until the late eighteenth and early nineteenth centuries, explaining as it did the belief that art should principally be concerned with naturalistic representation.

The late eighteenth- and early nineteenth-century cult of Romanticism encouraged exploration of the subjective or expressive human response to nature. For example, the oil-based landscapes by the German painter Caspar David Friedrich (1774–1840), or those of  the British artist J.M.W. Turner (1775–1851), were not simply concerned with achieving topographical accuracy, but were attempts to express the experience of the landscape upon human sense-perception.
The origins of abstraction in painting and sculpture in the late nineteenth century, and its adoption among the European avantgardes in the initial decades of the twentieth century, were concerned with art’s expressive potential independent of representation.
For example, having completed a version of his Black Square (Figure 13, p. 74), Malevich proclaimed ‘I have transfigured myself in the zero of forms’ – an aspiration which defied the traditions of naturalistic representation (Néret 2003: 50).
Of the aesthetic theories discussed, the institutional definition of art is widely referenced in relation to the contexts of postmodern culture of the later twentieth century. However, within Western traditions of art making, particular aesthetic ideas and theories have been fashionable at particular times and in particular contexts. Such ideas were (and remain) part of a broader cultural, political and economic framework in which various types and kinds of art were commissioned, produced and evaluated.