During the 1700s, Philadelphia was an unpleasant place in the summer. Malaria and yellow fever were rampant. There were no cures and no known ways to prevent infection. Most people of means tried to escape the city, if they could.

But in the scorching summer of 1776, scores of our country’s leading men remained behind closed doors in Philadelphia. They were kept there by their work. And what a monumental work it turned out to be.

The 56 leaders, representing all 13 British colonies, signed a declaration that would birth a great nation and illuminate the very future of humankind. It’s this Declaration of Independence that Americans celebrate each July 4.

The document’s first job was to officially announce to the world that all the colonies had decided to declare themselves free and independent states, absolved from any allegiance to Great Britain. That was momentous enough for the years ahead, since in order to make good on that declaration, the colonies would have to defeat the British in a war that stretched until 1783.

But the greater meaning of the Declaration — then as well as now — is as a statement of the conditions that underlie legitimate political authority and as an explanation of the proper ends of government.

The signers proclaimed that political power would spring from the sovereignty of the people, not a crowned hereditary monarch. This idea shook Europe to its very core.

The Declaration appealed not to any conventional law or political contract but to the equal rights possessed by all men and “the separate and equal station to which the Laws of Nature and nature’s God” entitled them.

What is revolutionary about the Declaration of Independence is not that a particular group of Americans declared their independence under particular circumstances. It’s that they did so by appealing to –and promising to base their particular government on — a universal standard of justice.

It is in this sense that Abraham Lincoln praised “the man who, in the concrete pressure of a struggle for national independence by a single people, had the coolness, forecast, and capacity to introduce into a merely revolutionary document, an abstract truth, applicable to all men and all times.”

Of course, it required another war to extend those rights to all Americans, but the fact that they were written down in the Declaration was crucial in 1865, in 1965 and remains so today as well.

“If the American Revolution had produced nothing but the Declaration of Independence,” wrote noted historian Samuel Eliot Morrison, “it would have been worthwhile.”

As Thomas Jefferson, lead author of the Declaration, put it in 1821, “The flames kindled on the 4th of July 1776, have spread over too much of the globe to be extinguished by the feeble engines of despotism; on the contrary, they will consume these engines and all who work them.”

Those flames, the flames of freedom and opportunity, continue to spread. That’s a truth worth celebrating on the Fourth — and all year ’round.