Thomas Hobbes, answered from the texts and cited to the page.
The laws of nature are, at their foundation, dictates of right reason concerning what conduces to the preservation of life. Right reason, properly understood, is not some infallible faculty lodged mysteriously in the mind, but the act of reasoning itself — a man's own true reasoning about actions which may conduce to his advantage or another's loss.1
Where no commonwealth exists, each man's own reason must serve as the measure, since there is no civil law to supply the common standard. The first and foundational law is this: seek peace when it can be had, and when it cannot, look for aid in war.2 From this first law all the rest are derived, as conclusions from a premise; the subsequent laws are nothing but instructions on the means of securing either peace or self-defence.
The second law follows immediately: a man ought to be willing, when others are so too, to lay down his right to all things, so far as peace and his own defence require it.3 For where every man holds a right to all things, invasion on one part and resistance on the other necessarily follow, and that is war — contrary, therefore, to the very end reason prescribes.4
Now there is a question about whether these dictates deserve the name of laws at all. Strictly speaking, a law is the word of him who by right hath command over others; the dictates of reason, considered in themselves, are properly theorems — conclusions about what conduces to conservation and defence.5 They become laws properly so called only when considered as delivered in the word of God, who by right commandeth all things.
For the theist, then, they are divine commands; for one who declines that assumption, they function as hypothetical imperatives, advice about the necessary means to an end every man already wills by the necessity of his nature. What they are not is the consent of nations, nor the agreement of the wisest peoples, nor any universal custom of mankind.
Passion and evil custom are no less universal than reason, and no less part of human nature; they cannot therefore supply the criterion.6 Reason alone is the same in all men, because all men agree in willing to be directed toward their own good — and it is reason, not passion, that declares the ways of peace where peace may be obtained, and of defence where it may not.
By right reason in men's natural state, I mean, not, as many do, an infallible Faculty, but the act of reasoning, that is, a man's own true Reasoning about actions of his which may conduce to his advantage or other men's loss.On the Citizen, pp. 77–78
The first law of nature (the foundation) is: to seek peace when it can be had; when it cannot, to look for aid in war.On the Citizen, pp. 77–78
the second law requires only that 'a man be willing, when others are so too, as far forth as for peace, and defense of himself he shall think it necessary, to lay down his right to all things.'Leviathan, p. 33
when divers men have right not only to all things else, but to one another's persons, if they use the same, there ariseth thereby invasion on the one part, and resistance on the other, which is war; and therefore contrary to the law of nature.The Elements of Law Natural and Politic, pp. 42–43
'These dictates of reason men use to call by the name of laws, but improperly. For they are but conclusions, or theorems concerning what conduceth to the conservation and defense of themselves, whereas law, properly, is the word of him that by right hath command over others.'Leviathan, p. 33
it is not agreed upon, who shall judge which nations are the wisest... it is not the consent of passion, or consent in some error gotten by custom, that makes the law of nature. Reason is no less of the nature of man than passion, and is the same in all men.The Elements of Law Natural and Politic, pp. 42–43