Burden of proof is an issue that is often muddled or misunderstood. I will not be addressing burden of proof in general; rather, I am going to focus on a particular situation.
There are many cases in which one side is arguing to allow a particular behavior, and the other side is arguing against it. The opponent asks, Why should this behavior be allowed?
There are myriad examples. Three rather prominent ones are, Why should gays be allowed to marry? Why should people under 18 be allowed to vote? Why should people be allowed to carry guns?
In some cases, the opponent will take this argument a little further. People don’t need to carry guns, so there is no reason to allow people to legally carry guns. Or, minors don’t need to vote. Most teenagers just care about Britney Spears’ latest song. They’re not interested in politics.
These questions misconstrue the concept of burden of proof. They do so because of a certain well-established human right: liberty.
It is widely agreed that, all else being equal, liberty is a good thing. In fact, it’s a great thing. The likes of John Locke and Thomas Jefferson argued for the fundamental rights of life, liberty, and property. (And if you think about it, these three rights can be compressed into the single supreme right of liberty.) Libertarianism (the philosophy, not just the political party) is founded on the concept of self-ownership, a synonym for liberty. Kantian Deontology dictates that we treat other people as ends rather than as means — that is, that we do not restrict their liberty. Even Utilitarianism strongly supports the idea of liberty, although not quite so severely: in most cases, liberty is conducive to maximizing utility.
From this, we can infer that liberty should be maximized whenever possible. Our justice system reflects this. Even laws that restrict liberty are ultimately intended to increase liberty overall. For example, our freedom is restricted in the case of murder because if we murder someone, that completely robs them of their liberty.
I expect it is widely agreed upon that, all else being equal, liberty is preferential to restriction. This can be seen anecdotally. Many of what are considered to be the most horrible of situations are based primarily upon a restriction of freedom: slavery, imprisonment, and even death are all excellent examples. The more restrictive it is, the more suffering it will entail.
Now that we have examined the topic of liberty, we are equipped to answer the first three questions. Why should gays be allowed to marry? Why should people under 18 be allowed to vote? Why should people be allowed to carry guns?
The response is, why not? Liberty is preferential to restriction. All else being equal, we ought to allow gays to marry. We ought to allow anyone to vote. We ought to allow people to carry guns. In fact, all else being equal, we ought to allow people to marry animals and carry nuclear weapons. So why don’t we?
We don’t allow people to marry animals or carry nuclear weapons because, in those cases, all else is not equal. Animals cannot offer consent to marriage, so the contract would be meaningless and to enforce it would be to restrict the freedom of the animal. Furthermore, this would nearly always be used as an excuse to reap the legal benefits of marriage without the difficulty of marrying an actual person — in very few cases would someone actually be in love with an animal. In the case of carrying a nuclear weapon, the risks far outweigh the benefits.
Even as obvious as those arguments were, it is still my side’s responsibility to make those arguments. I was arguing for the restriction of liberty. If those arguments had never been made, then we would be morally obligated to allow inter-species marriage and the carrying of WMDs. However, since those arguments were made, we have legitimate reason to restrict freedom in those cases.
Am I saying that no one has ever made an argument for why gay marriage should be illegal, or for why minors should not be able to vote, or for why guns should be banned? Not at all. Rather, I am addressing the specific argument that attempts to shift the burden of proof. The question is not why we should allow these things. The question is why we should NOT allow these things. By default, we are allowed to do anything. Liberty cannot be restricted without a good reason, and this is why the question of why we should allow something is not a legitimate one.