"A Modest Proposal" is a satire couched in the kind of logical, scientific language used in publications by the Royal Society. Swift was profoundly skeptical of the value of many of the proposals emanating from Britain's leading scientific academy. He satirizes its work in Gulliver's Travels when Gulliver reaches Laputa, a society in which, among other things, absent-minded scientists conduct pointless experiments such as attempting to extract sunbeams from cucumbers.
It's entirely appropriate, then, for Swift to use the rhetorical device of logos in the "Proposal," as this would've been how scientists of the Royal Society would have presented their findings. Examples abound:
The question therefore is, How this number shall be reared, and provided for? which, as I have already said, under the present situation of affairs, is utterly impossible by all the methods hitherto proposed.
This makes it seem that the author of the pamphlet has thought long and hard about the solution to this "problem"...
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