According to “formalist” Clive Bell, all and only those objects exhibiting significant form are art. But what makes some forms (combinations of lines, shapes, and colors) significant? Here, as I understand it, is Bell's answer:
1. We know significant form by its effect on us: it evokes (in the ideal observer), a “peculiar” aesthetic emotion. Yet,
2. We know these aesthetic emotions only as those triggered by our observation of significant form based on "unknown and mysterious" laws.
Unfortunately, Bell’s tight little circle brings us no closer to an appreciation of the difference between significant and insignificant (artless?) form. His central thesis that all and only those objects with significant form are art reduces to one of two apparently insignificant claims:
3. All and only those objects with some unknown, mysterious property are art. Or
4. All and only those objects that evoke some peculiar aesthetic emotional reaction are art.
In addition, some critics have found Bell to be unjustifiably elitist. Here's a review of The Intellectuals and the Masses which contains a brief poke at Clive Bell's inflated regard for his own tastes: