In his essay, "Science and Beauty," Isaac Asimov begins his argument with a quote from Walt Whitman's poem "When I Heard the Learn'd Astronomer." Whitman's poem presents a dichotomy between the cold, analytical view of the universe as presented by the astronomer, and the awe-inspiring beauty of the night sky when experienced directly.

This dichotomy forms the basis of Asimov's argument. He argues that science does not diminish the beauty of nature, but rather enhances it. Here's how Asimov structures his argument:

1. Paragraph 1 - Asimov begins by quoting Whitman's poem, thereby setting up the dichotomy between the scientific and the aesthetic view of the universe. This serves to introduce the central question of the essay: Does understanding the scientific principles behind natural phenomena diminish their beauty?

2. Paragraph 2 - Asimov acknowledges that many people would agree with Whitman's sentiment. This establishes common ground with readers who might initially be skeptical of Asimov's thesis.

In subsequent paragraphs, Asimov challenges this dichotomy by arguing that scientific understanding enhances rather than diminishes the aesthetic experience of nature. He provides examples from astronomy, chemistry, and biology to illustrate his point.

As for the unity and coherence of Whitman's poem, Whitman achieves this through the repetition of certain words and phrases, the use of parallel structure, and the progression of ideas from the specific to the general. The repetition of the word "when" at the start of each line creates a rhythmic pattern that unifies the poem. The parallel structure of the lines contributes to the coherence of the poem by creating a clear, logical progression of ideas. Finally, Whitman moves from the specific (hearing the astronomer speak) to the general (looking up at the stars), which helps to unify the poem and tie together its central themes.