An argument is a prose summary of the poem that is about to follow.
In "The Marriage of Heaven and Hell", William Blake begins with an argument. However, conventional poems like "Essay on Man" present arguments in prose and the rest of the work is in poetry- Blake,on the other hand, begins with an argument written in poetry and the rest of the poem is in prose.
This is significant because it demonstrates Blake's eccentricity as a writer and also his use of every literary and visual device possible to express his ideas through his works. In presenting the argument and poem in opposite forms from the conventional method, Blake presents opposition, which is what "The Marriage of Heaven and Hell" is about. He even uses different colors on each of his covers, which go against conventional norms.
William Blake is significant because although he was a neoclassical writer, he also represents the shift from conventional norms in writing to bending the rules. In doing so, he helps bridge the gap between Neoclassicism and Romanticism. In "The Marriage of Heaven and Hell", he is attacking anyone who buys into his conventional distinctions between good and evil. In writing in such a eccentric manner, Blake is setting an example of himself that he is thinking outside of the conventional way of thinking.