And because the condition of man. . . Is a condition of war of every one against every one, in which case every one is governed by his own reason, and there is nothing he can make use of that may not be a help unto him in preserving his life against his enemies; it followed 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 endured, there can be no security to any man, how strong or wise soever he is, of living out the time which nature ordinarily allowed men to live. And consequently it is a precept, or general rule of reason: that every man ought to endeavor peace, as far as he has hope of obtaining it; and when he cannot obtain it, that he may seek and use all helps and advantages of war. The first branch of which rule contained the first and fundamental law of nature, which is: to seek peace and follow it. The second, the sum of the right of nature, which is: by all means we can defend ourselves.

Leviathan