Actually, the Declaration of Independence is based on the political theories of John Locke, which Locke had written to justify the Glorious Revolution of 1688. Thomas Jefferson used Locke's logic to explain to the world several things:
- All people are born with certain natural rights. Locke had used the phrase "life, liberty and estate." Jefferson changed the last to "the pursuit of happiness."
- Governments are created by people for the specific right of protecting their natural rights.
- People have the right to change their government if/when it no longer protects their rights.
- The government of Great Britain, personified by the King of Britain, no longer protected the natural rights of the people of the colonies. Jefferson includes a virtual laundry list, which he called a "long list of abuses and usurpations" as ways in which the rights of the people of the colonies were no longer protected.
- Therefore they had the right to change their government, and now exercised that right to do so.
The Declaration thus explained to the world why the people of the (now) United States believed they had a right to be free from the government of Great Britain. Jefferson's opening lines indicate that the people of America were aware of the need to explain their position:
When in the Course of human events it becomes necessary for one people to dissolve the political bands which have connected them with another and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
The Declaration of Independence explained two related things about America's desire to be independent.
First, it explained the theoretical underpinnings of American independence. This is seen in the earliest parts of the Declaration. There, Jefferson explained why the colonies could theoretically have the right to pull away from England. This is where Jefferson explains about government existing by the consent of the governed and where he says the government must preserve the people's rights.
Second, it explained the specific ways in which England allegedly failed the colonies. After saying that a government has to preserve the rights of its people, Jefferson specifies ways in which the British government failed to preserve the colonists' rights.
In doing these things, Jefferson explained why the colonies could and should be independent.