A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.

What we know as the Second Amendment today was originally the fourth of twelve amendments proposed by the Joint Resolution of Congress in 1789. The purpose of the proposed amendments is described in the preamble.

"THE Conventions of a number of the States, having at the time of their adopting the Constitution, expressed a desire, in order to prevent misconstruction or abuse of its powers, that further declaratory and restrictive clauses should be added: And as extending the ground of public confidence in the Government, will best ensure the beneficent ends of its institution. RESOLVED by the Senate and House of Representatives of the United States of America, in Congress assembled, two thirds of both Houses concurring, that the following Articles be proposed to the Legislatures of the several States, as amendments to the Constitution of the United States, all, or any of which Articles, when ratified by three fourths of the said Legislatures, to be valid to all intents and purposes, as part of the said Constitution; viz. ARTICLES in addition to, and Amendment of the Constitution of the United States of America, proposed by Congress, and ratified by the Legislatures of the several States, pursuant to the fifth Article of the original Constitution."

This description from archives.gov gives some of the historical context in which the proposals were made:

"During the debates on the adoption of the Constitution, its opponents repeatedly charged that the Constitution as drafted would open the way to tyranny by the central government. Fresh in their minds was the memory of the British violation of civil rights before and during the Revolution. They demanded a "bill of rights" that would spell out the immunities of individual citizens. Several state conventions in their formal ratification of the Constitution asked for such amendments; others ratified the Constitution with the understanding that the amendments would be offered."

On September 25, 1789, the First Congress of the United States therefore proposed to the state legislatures 12 amendments to the Constitution that met arguments most frequently advanced against it. The first two proposed amendments, which concerned the number of constituents for each Representative and the compensation of Congressmen, were not ratified. Articles 3 to 12, however, ratified by three-fourths of the state legislatures, constitute the first 10 amendments of the Constitution, known as the Bill of Rights.

Interpretation of the Second Amendment for nearly two hundred years was seemingly a matter of historical reference held in the collective consciousness of the citizens and passed from generation to generation by teaching and action. The meaning of those 27 words was so well understood that there was no case law to test the boundaries or interpretation of the amendment.

Based on tradition predating the American Revolution, people had the right to have their own arms for their individual use and personal defense.
In an analogy to the Affordable Care Act, this is a pre-existing condition that is enumerated in the United States Constitution.

Today there is case law from the United States Supreme Court that relates to the Second Amendment.

D.C. vs Heller was decided in 2008 and held that the Second Amendment protects an individual right to possess a firearm unconnected with service in a militia, and to use that arm for traditionally lawful purposes, such as self-defense within the home.
Self-defense is a basic right, recognized by many legal systems from ancient times to the present, and the Heller Court held that individual self-defense is "the central component" of the Second Amendment right.
Read the full text at: http://www.supremecourt.gov/opinions/07pdf/07-290.pdf

In the 2010 McDonald vs Chicago decision, the Supreme Court held that the Fourteenth Amendment makes the Second Amendment right to keep and bear arms fully applicable to the States.
Read the full text at: http://www.supremecourt.gov/opinions/09pdf/08-1521.pdf

Blogs, books, magazines, politicians, talking heads, ... offer opinions about the Second Amendment ad nauseam. The "Court of Public Opinion" does not have judicial authority to render decisions that become case law.

Any Legislation can be passed into law. Concerned citizens or groups can challenge laws that seek to erode or restrict the Second Amendment and fight all the way to the Supreme Court for a decision.
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