The Declaration speaks of “truths” which are “self-evident.” What are these “truths”? Why are they called “truths”? What makes them “self-evident”?
The truths are listed in the Declaration of Independence.
Here are the truths Jefferson listed: (1) all men are created equal, (2) men are endowed by their Creator with certain unalienable rights, (3) among the rights that men have are the rights to life, liberty, and the pursuit of happiness, (4) governments are created to secure these unalienable rights, (5) governments get their powers from the consent of the governed, (6) when a form of government starts destroying people's rights, the people have the right to alter or abolish it and create a new government.
The "truths" Jefferson listed are the principles that he and many of the other American patriots believed in. The "truths" differed from the "truths" that monarchists believed in. Jefferson believed that his "truths" were "self-evident" (meaning that they were obvious and hard to disagree with) to educated, enlightened men like himself who had studied the works of John Locke, Jean Jacques Rousseau, Voltaire, and others who disagreed with monarchists.
Monarchists believed that kings and queens had the right to rule over people because the kings and queens were chosen by God. Kings, queens, and other rulers had been in charge of countries for hundreds of years. They frequently used religion to justify their power. If the kings and queens were chosen by God, then it would be hard for ordinary people to disagree with them.
It's important to remember that in Jefferson's time, most people were illiterate -- they could neither read nor write. Illiterate people get their information from other people and from observation, since they can't read. Kings, queens, popes, priests, rabbis, imams, military commanders, and other rulers simply told illiterate people what to believe and what to do. If people refused to obey, they were imprisoned, fined, physically punished, or even killed.
Jefferson's writings were designed to disagree with the rulers who based their power on religion. Jefferson told ordinary people that they had rights. This was a new concept for most people in Jefferson's time.
That all men are created equal and therefore have basic rights.
To anyone reading the Bible for themselves it was "self evident" that all people were created equal. Paul writes that in Christ there is no difference between male and female, between free or slave, between Greek and Jews. (Gal 3 :28) Clearly in the view of the Bible all men are created equal.
In Thomas Jefferson's time most people could read the Bible for themselves. One of the major impacts of the Great Awakening was an increase in literacy. In 1670 before the Great Awakening 60% of the population were literate. In 1760 just before the American Revolution 85% of the population was literate.
John Locke said "reason teaches all mankind would but consult it that being equal and independent no one or pose to harm anyone in life health liberty possessions." (Second Trieatise concerning Civl Government) Jefferson was greatly influenced by the writings of John Locke and agreed that it was logically that people had basic human rights to liberty.
John Locke in a previous essay had proven that no where in the Bible was the concept of the divine right of kings could be found. The power of the elite was based on traditions of the medieval church not Biblical authority or human reason.
So based on human reason and Biblical authority Jefferson stated it was self evident to anyone who studied the issue that people are created equal and have basic human rights.