Please explain this statement regarding Jonathan Swift's satire "A Modest Proposal."
Swift's satire would be impossible if he and his audience didn't share certain moral assumptions such as that cannibalism is evil and that injustice and exploitation should be eliminated. The satire depends on the audience sensing certain incongruities and finding the outlook and tone of the speaker morally inappropriate to the situation.
Jonathan Swift's satire "A Modest Proposal" appears to make a cool, logical, and rational argument in favor of consuming Irish babies for food. One question we can ask about the work is how we can determine that it is satirical, rather than an eminently practical solution to Irish famine.
There are two types of evidence that your quoted passage implies we can use in determining that the work is satirical. The first type of evidence is extrinsic to the work itself, and consists of our knowledge about the values and practices of Swift and his readers. The second type of evidence is intrinsic, found in the text itself.
On the level of extrinsic evidence, Jonathan Swift was an ordained priest in the Church of England, serving in the important position of Dean of the (Anglican) Dublin Cathedral. Cannibalism was regarded as barbaric in 18th century England, and there is no evidence that Swift condoned or practiced it.
Intrinsic evidence depends more on interpretation of tone. One example of how Swift signals his satiric intent is in his argument that his proposal would alleviate suffering as follows:
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 ...
The glaring contradiction between horror at women killing their own babies immediately followed by the suggestion that they kill and eat their babies or sell the babies for food makes the reader aware that Swift is not offering an actual solution to Irish poverty, but satirizing the absurdity of many of the solutions proposed in the past.