It's an Eighteenth Century concept. In the Age of Reason the universe often was conceived of as a machine, like a clock, which functioned properly when it was "well regulated." Sindawe's message is probably closest to that concept.
Although many of the previous suggestions are pretty good, they miss the essential recognition that the parts of the machine must themselves be perfect or else it would be impossible for the complete machine to be perfect.
In Eighteenth Century terms, within this new country the militia was conceived as a defensive organization drawn from the people and not from a class of professional soldiers such as the Hessians or other mercenaries. In the old world, wars were fought offensively as well as defensively by professionals. Of course the professionals in those countries had arms or were supplied with arms by the government, so there was no need for the people to have arms. And, in fact, an armed people in those old world countries was a danger to the government. Governments in the old world simply could not conceive of guaranteeing "the right to keep and bear arms" to their people. It would have been incomprehensible under a monarchical concept of government, for example, and suicidal too.
The United States was different from the old world in a great many ways and the Second Amendment is crucial to understanding the essential difference. Because this new country was committed to defending itself, and because its militia was any citizen in the country, and because the efffectiveness of the militia as a machine depended on the skills of the citizens who comprised the parts of the militia, the people had a right to keep arms (in other words, to own firearms, lances, spears, and whatever else could be used as a weapon) and to bear them (in other words, to carry them on their person and to use them at any time--with the assumption that they would not be used for illegal purposes, such as robbery or murder). Look at the statue of the Minute Man for one representation of the concept: he is an idealized citizen-soldier, holding his musket while he stands next to his plow, an embodiment of the basic component in a well-regulated militia.
In case anyone is interested, the Civilian Marksmanship Program was founded in 1903 by the U.S. government on exactly those principles. They worked. They were a major reason why this country was able to create effective military forces out of its citizen-soldiers in all wars through World War II: a majority of the U.S. population was familiar with guns and their use. The U.S. government encouraged them in those endeavors every way possible, because in case of war they could be trained fairly quickly: eight weeks of basic training followed by eight weeks of training in a specialty, and we had troops that were capable of performing at least competently. Although some of what I've said is subject to discussion or even argument, the basic concepts are accurate.
The concepts started to go out the window after the Kennedy and Martin Luther King assassinations, were dealt a fatal blow when the draft was discontinued (an awful blunder, I think), the country shifted towards a volunteer military (which implies a military made solely of professional soldiers and a military class), and growing numbers of people began to believe that there was no longer a need or a national benefit in people owning arms or becoming skilled in using them. These are all dreadful mistakes based on misunderstanding or ignorance, and they hurt the country. They also make the country's founding fathers and all of its Presidents up to Lyndon Johnson roll around in their graves and curse their descendants for screwing up their well-regulated machine.
I feel the need to add something of possible importance. What I've described had nothing to do with whether a President was a "liberal" or a "conservative." Most of the Presidents throughout our history owned and shot guns as a matter of course. For example, Eleanor Roosevelt (liberal wife of the liberal FDR, and one of my eternal heroes) shot revolvers as a hobby and carried one for self defense when she traveled. She would have thought that contemporary anti-gun liberals are just plain nuts and an embarrassment. Here's a photo of her in action: