In the satirical essay "A Modest Proposal," author Jonathan Swift certainly does mix a reasonable voice with a more absurd tone to convey his satire.
One example of his use of reason can especially be seen where he makes a list of six rational arguments to defend his satirical point of view. The first rational argument he lists is that consuming children would decrease the "number of Papists," meaning supporters of the Catholic Church, since Papists are the "principle breeders of the nation, as well as our most dangerous enemies." One reason why he says most of the nation is bred by Papists is because Ireland is predominantly a Catholic nation. A second reason why he says most of the nation is bred by Papists is because, due to religious views, Catholics have many children. He also says that Papists are the nation's enemies because Ireland was at the time being oppressed by England's Royalist supporters, who were Protestant and wealthy, leaving the Catholics to be the poorest and most oppressed and hated citizens of Ireland. Hence, Catholics were being treated as enemies of the nation even if they weren't truly enemies.
One of his most reasonably-sounding arguments is that, if children were sold for consumption, the "poorer tenants will have something valuable of their own," which they can use to help them out of their poverty and to "pay their landlord's rent."
Hence, all throughout the essay, Swift employs a voice of reason by making the argument sound rational. He then couples his rational argument with irony to create and emphasize his satire.