In District of Columbia v. Heller, the Supreme Court is anticipated to finally decide whether the Second Amendment is an individual or a collective right. This article is not about the textual and historical arguments on the basis of which the Court is likely to make its decision. My topic is more fundamental. Assuming that the Second Amendment protects an individual right, what purpose does it serve? What are the possible reasons that private arms possession is sufficiently valuable to deserve (...) constitutional protection? Because it was insufficiently sensitive to the variety of justifications available, the majority in Parker v. District of Columbia (the D.C. Circuit case appealed in Heller) failed to identify the purposes of the Second Amendment. The passing comments it made were compatible with a large number of very different justifications. Second Amendment advocates have also been surprisingly muddled on the issue. This confusion has gone unnoticed because no one has, until now, offered a philosophically rigorous account of the justifications available and the important distinctions between them. Clarity about the value of private arms possession is essential for determining the scope of the Second Amendment under an individual right interpretation - a project that lower courts will be forced to undertake if Parker is affirmed. Courts commonly interpret the scope of a constitutional right in light of the interests the right protects. For this reason, they need a clear conception of why individuals have an interest in private arms possession. I offer this article as a first, but crucial, step toward answering this question. (shrink)