Final answer:
The rhetorical device used in the phrase is parallelism, which emphasizes the idea of simplification and moderation by repeating a certain structure.
Step-by-step explanation:
The passage "Instead of three meals a day, if it be necessary eat but one; instead of a hundred dishes, five; and reduce other things in proportion." utilises the rhetorical device known as parallelism or parallel structure. This device involves the repetition of the same pattern of words to show that two or more ideas have the same level of importance. The sentence shows parallelism by repeating the structure "Instead of...; if it be necessary...; instead of...; and reduce other things in proportion." This pattern emphasizes the idea of simplification and moderation.
Other common rhetorical devices include metaphors and similes, which are not present in the given example. These devices compare two things, with metaphors stating they are the same, and similes using "like" or "as" to make a comparison. The use of parallelism in the passage achieves balance, calls attention to the ideas, and makes the statements more memorable, showing the author's prowess in language and persuasion.