In the 19th century, the Impressionists broke hundreds of years of convention by painting their immediate impressions of whatever scene lay before them, with emphasis on capturing the specific qualities of light as it existed in the moment. When these painters emerged onto the art scene, the Impressionists were considered to be little more than rogues, upsetting a great and grand tradition of art. This new style of painting quickly led to an avalanche of other new styles, all of which now fall into the Modern art movement.
At the outset of the 20th century, the Expressionists were part of a larger cultural movement that also included writers and dancers. The Expressionists painted less of what they saw and more of what they felt. The paintings of the Expressionists were wildly subjective, portraying raw emotions and abstracted picture planes. These textural, visceral, immediate paintings were often colorful and crude.
The Cubists, Dadaists and Surrealists were all very different artists, dealing with similar themes in dissimilar ways. While the Cubists took the two-dimensional picture plane and made it into a three-dimensional conundrum, the Dadaists tried to disassemble the very foundations of art by painting art nonsense (sometimes called "anti-art"). Meanwhile, the Surrealists painted from the subconscious, bending time, space and reason, creating loopy and disturbing images of real and unreal. Although these different movements were very unlike each other in product, the intellectualism and sensationalism driving each movement was the same. These movements reinforced a new standard in art that required artists to break conventions and expectations.
When it's taken in an overall context, the true basis of Modern painting seems to be intellectual lawlessness. The new rule is, there are no rules. It is the true achievement of the Modern painters, and the one and only real principle of Modern painting. Literally, do whatever you want, but preferably, do it in a way that is different, unexpected and outrageous.