Thomas Hobbes, answered from the texts and cited to the page.
The question is large, but the answer follows from a single structural observation: men are by nature equal, and from that equality all the trouble flows. Equal in body and mind — for even the weakest can kill the strongest, by secret machination or confederacy with others, and every man is contented with his own share of wit, which is itself the surest sign of equal distribution.1
From this equality arises equality of hope in the attaining of our ends. When two men desire the same thing, which they cannot both enjoy, they become enemies; and in the way to their end, which is principally their own conservation, each endeavours to destroy or subdue the other.2 The invader who plants and builds must expect others to come with forces united to dispossess him; and the invader again is in the like danger of another.3
Three causes of quarrel follow from this structure: competition, diffidence, and glory.4 Competition drives men to invade for gain; diffidence drives them to invade for safety; glory drives them to invade for reputation — for a word, a smile, a different opinion, any sign of undervalue, however slight.5 Men in company without a common power to overawe them all have no pleasure in one another's company, but a great deal of grief; each man looks that his companion should value him at the same rate he sets upon himself, and upon all signs of contempt naturally endeavours to extort a greater value by damage.6
The estate that follows from this natural liberty is the estate of war — not the act of fighting merely, but the known disposition to fight during all the time there is no assurance to the contrary, as foul weather consists not in a shower or two but in the inclination thereto of many days together.7 In such a condition there is no place for industry, because the fruit thereof is uncertain; no culture of the earth, no navigation, no commodious building, no arts, no letters, no society; and which is worst of all, continual fear and danger of violent death, and the life of man, solitary, poor, nasty, brutish, and short.8
One point requires emphasis, because it is commonly misread. I do not say that men are by nature malicious in the way a wolf is malicious. The passions that arise from animal nature are not themselves evil, though the actions that proceed from them sometimes are.9 The vainglorious man who seeks unnecessary power over his fellows is moved by a belief — a false estimate of his own strength, an act of imagination — not by pure predatory appetite.10
Even the contest over scarce resources is, on my account, unnecessary if society is well ordered; throughout De Cive I assumed that such contest need not arise where the commonwealth is properly constituted.11 The right of every man to every thing — natura dedit omnia omnibus — is in effect no better than if no man had right to anything, since another as strong or stronger hath right to the same; and it is this structural absurdity, not wickedness of heart, that makes the state of nature the state of war.12 That is why the remedy is not moral reformation but sovereign power: a common power able to keep them all in awe.
For they see their own wit at hand, and other men's at a distance. But this proveth rather that men are in that point equal, than unequal. For there is not ordinarily a greater sign of the equal distribution of anything than that every man is contented with his share.Leviathan, pp. 157–158
From this equality ariseth equality of hope in the attaining of our ends. And therefore, if any two men desire the same thing, which nevertheless they cannot both enjoy, they become enemies; and in the way to their end, which is principally their own conservation, and sometimes their delectation only, endeavour to destroy or subdue one another.Leviathan, pp. 157–158
if one plant, sow, build, or possess a convenient seat, others may probably be expected to come prepared with forces united, to dispossess and deprive him, not only of the fruit of his labour, but also of his life or liberty. And the invader again is in the like danger of another.Leviathan, pp. 157–158
So that in the nature of man we find three principal-causes of quarrel: first, competition; secondly, diffidence; thirdly, glory.Leviathan, pp. 158–159
The first maketh men invade for gain; the second, for safety; and the third, for reputation. The first use violence to make themselves masters of other men's persons, wives, children, and cattle; the second, to defend them; the third, for trifles, as a word, a smile, a different opinion, and any other sign of undervalue.Leviathan, pp. 158–159
For every man looketh that his companion should value him at the same rate he sets upon himself, and upon all signs of contempt, or undervaluing, naturally endeavours, as far as he dares… to extort a greater value from his contemners, by damage, and from others, by the example.Leviathan, pp. 157–158
the nature of war consisteth not in actual fighting, but in the known disposition thereto during all the time there is no assurance to the contrary. All other time is PEACE.Leviathan, pp. 158–159
In such condition there is no place for industry, because the fruit thereof is uncertain, and consequently, no culture of the earth, no navigation… no arts, no letters, no society, and which is worst of all, continual fear and danger of violent death, and the life of man, solitary, poor, nasty, brutish, and short.Leviathan, pp. 158–159
For the passions which arise from animal nature are not themselves evil, though the actions that proceed from them sometimes are, namely, when they are harmful and contrary to duty.On the Citizen, pp. 19–20
In his case, the will to do harm derives from conceit [inanis gloria, vainglory] and over-valuation of his strength… The role of belief is quite explicit in the second case Hobbes discussed, 'intellectual dissension' such as wars 'between different sects of the same religion and factions of the same country.'On the Citizen, pp. 19–20
Hobbes was in fact very unwilling to suppose that, in the world as presently constituted, there could be a genuine struggle over scarce resources: throughout De Cive he assumed that contest over resources is unnecessary if a society is well ordered.On the Citizen, pp. 19–20
that right of all men to all things, is in effect no better than if no man had right to any thing. For there is little use and benefit of the right a man hath, when another as strong, or stronger than himself, hath right to the same.The Elements of Law Natural and Politic, pp. 41–42