Hobbes

During the time men live without a common power to keep them all in
awe, they are in that condition which is called war; and such a war as
is of every man against every man. In this state any person has a
natural right to do anything to preserve his own liberty or safety,
and life is "solitary, poor, nasty, brutish, and short." ...in the
international arena, states behave as individuals do in a state of
nature.

Within the state of nature there is no injustice, since there is no
law, excepting certain natural precepts, the first of which is "that
every man ought to endeavour peace, as far as he has hope of obtaining
it". ; and the second is "that a man be willing, when others are so
too, as far forth as for peace and defence of himself he shall think
it necessary, to lay down this right to all things; and be contented
with so much liberty against other men as he would allow other men
against himself". . From this, ...the way out of the state of nature
[is] into civil government by mutual contract. (bellum omnium contra
omnes)

Locke

The state of Nature has a law of Nature to govern it, and that law is
Reason. ...reason teaches that no one ought to harm another in his
life, health, liberty or possessions; and that transgressions of this
may be punished. This view of the state of nature is partly deduced
from Christian belief (unlike Hobbes, whose philosophy is not
dependent upon any prior theology): the reason we may not harm another
is that we are all the possessions of God and do not own ourselves.

Rousseau

...Hobbes was taking socialized persons and simply imagining them
living outside of the society in which they were raised. [Rousseau]
affirmed instead that people were naturally good. Men knew neither
vice nor virtue since they had almost no dealings with each other.
Their bad habits are the products of civilization. Nevertheless the
conditions of nature forced people to enter a state of society by
establishing a civil society.

John Rawls

[We must use] ...an artificial state of nature. [One that] places
everyone in the original position. The original position is a
hypothetical state of nature used as a thought experiment: People in
the original position have no society and are under a veil of
ignorance that prevents them from knowing how they may benefit from
society. They do not know if they will be smart or dumb, rich or poor,
or anything else about their fortunes and abilities. ...people in the
original position would want a society where they had their basic
liberties protected and where they had some economic guarantees as
well. If society were to be constructed from scratch through a social
agreement between individuals, these principles would be the expected
basis of such an agreement. Thus, these principles should form the
basis of real, modern societies since everyone should consent to them
if society were organized from scratch in fair agreements.

http://en.wikipedia.org/wiki/State_of_nature