Thomas Paine's writings are, taken as a whole, an iconic expression of not only the specific arguments in favor of American independence and democracy, but of the key elements of Enlightenment thinking that have in some senses become permanent, still affecting the world today.
Common Sense makes the point first that monarchical forms of government are obsolete, and second, that there is no reason the American colonies should continue to be ruled by Great Britain. The majority of the world's population today would probably agree reflexively with the first point. Great Britain itself has evolved into a state today in which the monarch is a mere figurehead, unlike in the eighteenth century when the King, though even at that time not an absolute monarch as in other countries, did have genuine, but restricted, power. What is particularly convincing in Common Sense is Paine's outline of the history of England, in which monarchy did not create stability but instead resulted in long, bloody conflicts such as the Wars of the Roses. Paine discredits the whole line of English monarchs going back to the Norman Conquest, describing William the Conqueror as a usurper with no legitimate claim to ruling England, landing there and taking over the country by force with an armed "banditti." Moreover the idea of hereditary succession, he says, is absurd given that even if a given monarch is just, there is no guarantee that his offspring will be the same way. It also makes no sense for succession to occur when the heir to the throne is a child and a Regency is established where the regent can do all sorts of mischief; nor is their any logic in a monarch who is only a youth of twenty-one to rule over a country made up largely of people much older and wiser than he's likely to be.
Paine's reasoning in favor of breaking with the Crown is persuasive. He correctly states that Britain only defended the Colonies in the recent Seven Years's War because it was in Britain's interest to weaken its own enemy, France. He points out that as long as Britain controls America, the constant wars among the European countries themselves, which had been going on for centuries, would be a detriment to America and would end up harming Americans rather than aiding them.
Of particular interest is the point he makes about the ethnic composition of the colonies. At this time (1776), he states, a majority of the population of Pennsylvania, for instance, is not of English descent, but has its ancestry in other European countries. Therefore, he says, all Europe, not England, is the "parent" of America. Unfortunately Paine does not mention the Native Americans or African Americans, though elsewhere he wrote forcefully against slavery and about the enormous injustice done to the enslaved people of African descent.
Rights of Man is a kind of extension of the more general principles against monarchy and any non-representative government Paine put forward in Common Sense. It is also largely a response, or rebuttal, to Edmund Burke's Reflections on the Revolution in France. Though Burke does make some valid points in his famous and seminal work, Paine correctly skewers Burke's assertion that the English people had explicitly rejected the right to choose their own form of government when Parliament declared its loyalty to William and Mary at the time of the Glorious Revolution. Paine forcefully states that there never has been, nor will there ever be, any Parliament or group of legislators or any "description of men" who have the right to decide how the world will be governed for all time.
In Right of Man Paine also, and perhaps even more significantly, writes about the general state of poverty in which most of the European population at that time was forced to live. In every major European city, he states, there is a group of people known as "the mob," an underclass who are the victims of the huge gap between rich and poor and the unfair, undemocratic governmental systems of Europe. In writing of the Gordon Riots in London in 1780, Paine states that "even the people who commit such acts should have some claim on our consideration." Paine was ahead of his time in recognizing the unfairness and dysfunction of the political and economic establishment, whether in the Colonies as still governed by Britain, or in Britain itself, or even Europe as a whole.