In Leviathan, Thomas Hobbes's central idea about self-preservation is expressed in the section he devotes to what he calls "the right of nature," which he defines in the following terms:
The Right of Nature, which Writers commonly call Jus Naturale, is the Liberty each man hath, to use his own power, as he will himself, for the preservation of his own Nature; that is to say, of his own Life; and consequently, of doing any thing, which in his own Judgement, and Reason, he shall conceive to be the aptest means thereunto.
This is as realistic as any statement of rights can be. The problem with any assertion of rights, as Hobbes clearly sees, is that what is called a "right" can always be defeated in practical terms by superior force. There is no point in saying that every man has a right self-preservation if another man is in fact able to destroy him. Therefore, Hobbes says, in the state of war that is the natural condition of humanity, idealistic statements of what one would like human rights to be are meaningless:
It followeth, that in such a condition, every man has a Right to every thing; even to one another's body. And therefore, as long as this natural Right of every man to every thing endureth, there can be no security to any man, (how strong or wise soever he be) of living out the time, which Nature ordinarily alloweth men to live.
The Leviathan, according to Hobbes, is therefore the only effective guarantor of rights. He alone can have such great power that he can prevent any one of his subjects from encroaching upon the rights of another. Hobbes is therefore entirely realistic in his view that a statement of the right to self-preservation, or any other right, is only meaningful if it is backed up by a legal power that is both willing and able to enforce that right on behalf of those too weak to enforce it themselves.