Answer:
In the excerpt from Essays in the Art of Writing, Robert Louis Stevenson uses both implied and explicit comparisons to illustrate the unique challenges and opportunities of the literary artist. One explicit comparison is the likening of literature's material to "blocks, rudely conceived for the purpose of the market or the bar." Here, Stevenson directly compares the words used in literature to simple, unrefined building blocks, suggesting that they lack the flexibility and nuance of materials used in other arts such as sculpture or painting. This comparison emphasizes the limitations faced by writers, who must construct their works using a limited vocabulary.
Stevenson also employs an implied comparison when he writes that "other arts obtain relief, continuity, and vigour: no hieroglyphic touch, no smoothed impasto, no inscrutable shadow, as in painting; no blank wall, as in architecture." Here, he suggests that literature lacks the ability to use techniques commonly employed in other art forms to create interest and meaning. By contrasting literature with painting and architecture, he implies that these other arts possess qualities that are absent in literature, further emphasizing the unique challenges faced by literary artists.
Overall, Stevenson's comparisons highlight both the limitations and opportunities of literature as an art form, emphasizing the necessity for writers to craft their works with precision and creativity using a limited set of tools.