What prevents written history, including primary sources, from being completely objective? What aspects of history do people disagree about? What factors cause these disagreement?

Hich line in this excerpt from jonathan swift's "a modest proposal" uses the rhetorical device of irony? and secondly, (there being a round million of creatures in humane figure throughout this kingdom, whose whole subsistence put into a common stock, would leave them in debt two million of pounds sterling), adding those who are beggars by profession, to the bulk of farmers, cottagers and labourers, with their wives and children, who are beggars in effect; i desire those politicians who dislike my overture, and may perhaps be so bold to attempt an answer, that they will first ask the parents of these mortals, whether they would not at this day think it a great happiness to have been sold for food at a year old, in the manner i prescribe, and thereby have avoided such a perpetual scene of misfortunes, as (they have since gone through, by the oppression of landlords, the impossibility of paying rent without money or trade, the want of common sustenance, with neither house nor cloaths to cover them from the inclemencies of the weather,) and the most inevitable prospect of intailing the like, or greater miseries, upon their breed for ever. i profess, in the sincerity of my heart, that( i have not the least personal interest in endeavouring to promote this necessary work, having no other motive than the publick good of my country,) by advancing our trade, providing for infants, relieving the poor, and giving some pleasure to the rich.( i have no children, by which i can propose to get a single penny; the youngest being nine years old, and my wife past child-bearing. reset next)