In Swift's "A Modest Proposal," the foundation of the work is what you refer to as the "reasonable voice" of the narrator. This sets up everything else in the work, including the irony, which contibutes so greatly to the essay's success. Nothing the narrator says would work if the narrator did not appear to be reasonable. His argument is based on reason.
The narrator begins by establishing how much he cares about the Irish people. This is vital. His argument must seem to be for the Irish people, not against them. For instance, he writes:
There is likewise another great advantage in my scheme, that it will prevent those voluntary abortions, and that horrid practice of women murdering their bastard children, alas, too frequent among us, sacrificing the poor innocent babes.
This is a reasonable, as well as an honorable goal.
Once he is established as reasonable and as being on the side of the Irish people, his modest proposal (which is anything but modest), once it is revealed, creates the irony which dominates the essay. The narrator remains rational and unemotional for the remainder of the essay, using facts and statistics as one might use to solve a math or science problem.
In this essay, Swift is emphasizing his satire by adopting such a reasonable tone of voice as he writes. This makes his essay seem serious. The seriousness adds to the satire.
Some examples of his reasonable voice might include:
- He uses statistics. He talks about how many people live in Ireland. He quite seriously calculates how many children might be made available and how much money they would cost their parents to raise, etc.
- He cites authorities -- like quoting sources in a modern news article. He claims to have interviewed merchants to find out how feasible his plan is.
I am not really clear on what you are asking. Are you asking about how Swift uses the voice of the "authorial persona" to narrate this essay?
This essay is about the strongest example of satire that you can find. Under the guise of a person with authority (authorial persona), Swift "proposes" his solution for overpopulation in Ireland. As the essay unfolds, he gives information about overpopulation and the reader believes he/she is reading a social essay that is going to offer a worthwhile solution to overpopulation, infant mortality, abortion, etc., until you come to this line:
I shall now therefore humbly propose my own thoughts, which I hope will not be liable to the least objection.
I have been assured by a very knowing American of my acquaintance in London, that a young healthy child well nursed is at a year old a most delicious, nourishing, and wholesome food, whether stewed, roasted, baked, or boiled; and I make no doubt that it will equally serve in a fricassee or a ragout.
We then learn that the "modest proposal" is that the children be used for food! And this is not a modest proposal, it is an outrageous proposal, thus the satire, because Swift is using something outrageous to bring our attention to the problem.
Read the analysis right here on eNotes.
If this is not what you are asking about, please re-post your question.