What pieces of evidence support Swift's claims in "A Modest Proposal"?
As the other educator has pointed out, it is critical to acknowledge that Swift's piece is a satirical one; thus, the evidence within this essay should not be misconstrued as literal evidence, but taken as clever argumentation intended to comment on heartlessness toward the poor.
In the essay, Swift suggests that the Irish may resolve their economic issues by consuming children as food. He uses made-up calculations to illustrate his point:
The number of Souls in this Kingdom being usually reckoned one million and a half, of these I calculate there may be about two hundred thousand couple whose wives are breeders; from which number I subtract thirty thousand couples who are able to maintain their own children... there will remain an hundred and seventy thousand breeders. I agains subtract fifty thousand for those women who miscarry, or whose children die by accident or disease with the year. There only remains one hundred and twenty thousand children of poor parents annually born.
This is an appeal to logos through the evidence that it is numerically "practical" to use children as sustenance, with Swift proposing that twenty thousand of these children be kept for breeding and the remaining hundred be sold as food throughout the kingdom. Again, Swift turns to numbers to back up his claims, speculating on everything from the number of meals a child may provide to his or her average weight in flesh to the financial costs of nursing poor children. Swift continues to play a "numbers game" throughout the essay to exaggerate the absurdity of what he is suggesting.
It is of great importance from the beginning that we take A Modest Proposal for the satirical work that it is. His being absurd in order to make a point.
One of his claims is that this proposal will
"prevent voluntary abortions, and that horrid practice of murdering ... bastard children!"
He supports this claim by giving hypothetical and numerical evidence. He calculates that "120,000 children of poor parents annually born" plague the society. These could "be reserved for breed..." and "offered for sale to the persons of quality and fortune through the kingdom."
He further gives evidence to the claim for selling children that he has
"been assured by a very knowing American... that a young healthy child well nursed at a year old [is] a most delicious, nourishing, and wholesome food."
Thus, he tries to imply that he has an expert source that would support his overall idea to eat children.
He provides evidence of discussion with merchants regarding the prices of what the different proportions of the child could yield for the profit of the poor.
He also later cites a time in history when after young persons had been put to death, their bodies were used for profit.
Swift uses statistical, historical, and authoritative types of evidence yet each of his arguments are weak and ill-founded in reality.