This is something that I don't encounter often, but recently I've encountered a few people who don't seem to even understand the concept of rights. I've encountered people who believe that a right to life doesn't even exist, which is just a bizarre view to hold. Apart from the fact that these arguments belie a misunderstanding of rights, these arguments can also be used to argue that the right to bodily autonomy doesn't exist, which pro-choice people need in order to justify their claim that abortion is morally permissible.
The first person I encountered quoted Robert Heinlein, the author of Starship Troopers. Now, I've never read the book, nor have I seen the movie. So I don't know if this is Heinlein's actual position or if he placed this on the lips of one of the fictional characters in the book/movie, but the quote is as follows:
"Life? What 'right' to life has a man who is drowning in the Pacific? The ocean will not hearken to his cries. What 'right' to life has a man who must die to save his children? If he chooses to save his own life, does he do so as a matter of 'right'? If two men are starving and cannibalism is the only alternative to death, which man's right is 'unalienable'? And is it 'right'?"
So first we must ask: what are rights? There are different theories about ethics. I, personally, take a deontological view of ethics, which just means that I believe that actions are right and wrong in and of themselves. According to the Stanford Encyclopedia of Philosophy, rights are: "entitlements (not) to perform certain actions, or (not) to be in certain states; or entitlements that others (not) perform certain actions or (not) be in certain states." In short, rights are duties to perform or not to perform certain actions, and not to have another person perform an action against me.
So who are the subjects of rights? Persons are the subjects of rights. It makes no sense to say that if a lion kills me, it has violated my rights because it is only acting out of pure instinct (it is not wrong for a lion to kill me if I tresspass on its lair, but it is wrong for a person to kill me if I tresspass on their property unless I am threatening them). The reason that persons are the subjects of rights is because persons have the ability to recognize right from wrong and act accordingly. 
The right to life, as I have argued elsewhere, is a negative right not to be killed unjustly. Since persons are the only subjects of rights, then the first part of Heinlein's quote is, frankly, unintelligible. Oceans are not the subjects of rights, so if I am in the ocean of my own volition and I drown, I have not been wronged by the ocean. The ocean is not consciously setting out to drown me, and even if it was (like animals can do), it doesn't recognize that murder is an immoral act. I am only wronged if another person put me in the ocean to drown me.
Additionally, the fact that there may be some difficulties does not prove a view incorrect. It just shows that a careful discussion of rights and duties needs to be had. But the rest of Heinlein's quote really isn't problmatic to a view of a right to life. When a person has children, that brings with it certain obligations (such as the obligation to provide for their basic necessities). It also brings with it an obligation to protect his/her children with their lives. This does not mean they don't have a right to life, it just means that they have other obligations. So if someone is threatening a man's children and he jumps in front of a bullet and dies instead, the murderer has violated his right to life by killing him (and his childrens' by threatening them), but he has lived up to his obligation as a father by protecting his children.
And as for cannibalism, this is also immoral. No one has the right to kill and eat another. One could conceivably give up his own life to save the other out of necessity, but if one willingly sacrifices himself then his right to life has not been violated (suicide is another matter, and beyond the scope of this essay to discuss).
There is much more to be said about the concept of rights and duties. I have offered only a brief sketch here. In the next part, I'll be responding to a few other misconceptions that I've encountered about what a right or a duty is, and whether or not a right to life actually exists.
 Philosophers who have affected my view of morality include (but are not limited to), and in chronological order, are: Aristotle, Thomas Aquinas, Immanuel Kant, C.S. Lewis, William Lane Craig, and J.P. Moreland.
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