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I think it is quite clear that Swift deliberately sets out to shock and horrify his audience with this excellent example of satire. Obviously the most shocking element of this treatise comes when Swift says:
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 ragout.
Swift makes his so-called "modest proposal" all the more shocking by writing it in a reasonable, calculated tone that seeks to find a practical solution to the Irish famine. Note how he has carefully presented himself as a concerned and practical citizen wanting to do his part to suggest a way of helping the famine victims. His objective and sensitive tone shows that really Swift is protesting against a view of humanity that treats humans as mere numbers. In thinking the unthinkable, Swift tries to show his audience how they have failed to do anything about the tremendous tragedy that is occurring so close to them.