In Search of the Republic--25
Although men live in a state of nature before forming governements, they are not without law. The state of nature has a law of nature.
Locke does not elaborate much on this law of nature even though the concept has a rich history going back at least to Cicero. He defines the law of nature as reason. And the most fundamental conclusion that reason teaches is each member of the human race must preserve himself. Moreover, every man must preserve every other man by not impairing another's right to life, liberty, or property:
"But though this be a state of liberty, yet it is not a state of licence: though man in that state have an uncontrollable liberty to dispose of his person or possessions, yet he has not liberty to destroy himself, or so much as any creature in his possession, but where some nobler use than its bare preservation calls for it. The state of nature has a law of nature to govern it, which obliges every one: and reason, which is that law, teaches all mankind, who will but consult it, that being all equal and independent, no one ought to harm another in his life, health, liberty, or possessions: for men being all the workmanship of one omnipotent, and infinitely wise maker; all the servants of one sovereign master, sent into the world by his order, and about his business; they are his property, whose workmanship they are, made to last during his, not one another's pleasure: and being furnished with like faculties, sharing all in one community of nature, there cannot be supposed any such subordination among us, that may authorize us to destroy one another, as if we were made for one another's uses, as the inferior ranks of creatures are for ours. Every one, as he is bound to preserve himself, and not to quit his station wilfully, so by the like reason, when his own preservation comes not in competition, ought he, as much as he can, to preserve the rest of mankind, and may not, unless it be to do justice on an offender, take away, or impair the life, or what tends to the preservation of the life, the liberty, health, limb, or goods of another."
The second fundamental law of nature is the right to use force, including lethal force, to protect one's life, liberty, and property.
"And that all men may be restrained from invading others rights, and from doing hurt to one another, and the law of nature be observed, which willeth the peace and preservation of all mankind, the execution of the law of nature is, in that state, put into every man's hands, whereby every one has a right to punish the transgressors of that law to such a degree, as may hinder its violation."
According to Locke, one may use lethal force even against robbery. He argues that when a robbery puts a victim under absolute authority in order to take property, this implies a threat to one's life as well.
Not all men, however, are content to live at peace according to reason. Some try to subject others and take they life, liberty, or property. Locke calls this a state of war.
"Men living together according to reason, without a common superior on earth, with authority to judge between them, is properly the state of nature. But force, or a declared design of force, upon the person of another, where there is no common superior on earth to appeal to for relief, is the state of war."
To prevent a state of war and to live more securely, men form themselves into societies with civil government.