Hobbes's greatest work, Leviathan (published in 1651), pursues a theme that had obsessed him for more than twenty years: the evils of civil war and the anarchy by which it would be accompanied. Nothing could be worse than life without the protection of the state, Hobbes argued, and therefore strong government is essential to ensure that we do not lapse into the war of all against all.
But why did Hobbes believe that the state of nature would be so desperate, a state of war, a state of constant fear and danger of a violent death? The essence of Hobbes's view is that, in the absence of government, human nature will inevitably bring us into severe conflict. For Hobbes, then, political philosophy begins with the study of human nature.
Hobbes suggests that there are two keys to the understanding of human nature. One is self-knowledge. Honest introspection tells us a great deal about what human beings are like: the nature of their thoughts, hopes, and fears. The other is knowledge of the general principles of physics. Just as to understand the citizen (the individual in political society) you have to understand human nature; Hobbes believed, as a materialist, that to understand human nature you must first understand 'body' or matter, of which, he urged, we are entirely composed.

For our purposes,the most important aspect of Hobbes's account of matter is his adoption of Galileo's principle of the conservation of motion. Prior to Galileo,philosophers and scientists had been puzzled by the question of what kept objects in motion.By what mechanism,for example,does a cannon-ball remain in flight once it has been fired?Galileo's revolutionary answer was to say that this was the wrong question. We should assume that objects will continue to travel at a constant motion and direction until acted on by another force. What needs to be explained is not why things keep going, but why they change direction and why they stop. In Hobbes's lifetime this view was still a novelty, ?and he pointed out, defied the common-sense thought that,just as we tire and seek rest after moving,objects will naturally do this too. But the truth, he claims,is that 'when a thing is in motion, it will eternally be in motion, unless somewhat els stay it'(Leviathan,87). This, he thought, was true for us too. Becoming tired and desiring rest is simply to have a different motion act upon us.

So the principle of the conservation of motion was used by Hobbes in developing a materialist, mechanist view of human beings. The broad outlines of this account are laid out in the introduction to Leviathan: 'What is the Heart, ?but a Spring; and the Nerves but so many Strings; and the Joynts, but so many Wheeles, giving motion to the whole Body...?' (p. 81). Thus human beings are animated through motion. Sensation, for example, is a 'pressing'on an organ. ?Imagination is a 'decaying relic'of sensation. A desire is an 'internal motion towards an object'. All of this is meant quite literally.?

The importance of the theory of the conservation of motion is that with it Hobbes paints a picture of human beings as always searching for something, never at rest. There is no such thing as perpetuall Tranquillity of mind while we live here; because Life it selfe is but Motion, and can never bewithout Desire'(Leviathan, 129-30). Human beings, Hobbes argues, seek what he calls 'felicity', continual success in achieving the objects of desire. It is the search to secure felicity that will bring us to war in the state of nature. Ultimately, Hobbes thought, our fear of death would bring human beings to create a state. But without a state, in the state of nature, Hobbes thought that the search for felicity would lead to a war of all against all. Why did Hobbes think this?

One clue can be found in Hobbes's definition of power: one's 'present means to obtain some future apparent Good'(Leviathan, 150). So to be assured of achieving felicity one must become powerful. Sources of power, Hobbes claims include riches, reputations, and friends, and human beings have 'a restlesse desire of Power after power, that ceaseth onely in Death'(Leviathan, 161). This is not only because humans can never reach a state of complete satisfaction, but also because a person 'cannot assure the power and means to live well, which he hath present, without the acquisition of more' (Leviathan, 161). For others will also seek to increase their power, and so the search for power, is by its nature, competitive.

Everyone's natural, continual, attempt to increase power-to have riches and people under one's command-will lead to competition. But competition is not war. So why should competition in the state of nature lead to war? An important further step is Hobbes's assumption that human beings are by nature'equal. Anassumption of natural equality is often used in political and moral philosophy asa basis for the argument that we should respect other people, treating one another with care and concern. But for Hobbes the assumption is put to a quite different use, as we might suspect when we see how he states the point: we are equal in that all humans possess roughly the same level of strength and skill, and so any human being has the capacity to kill any other. 'The weakest has strength enough to kill the strongest, either by secret machination or by confederacy with others' (Leviathan, 183).

To this Hobbes adds the reasonable assumption that in the state of nature there is a scarcity of goods, so that two people who desire the same kind of thing will often desire to possess the same thing. Finally, Hobbes points out that no one in the state of nature can make himself invulnerable against the possibility of attack. Whatever I possess, others may desire, and so I must constantly be on my guard. Yet even if I possess nothing I cannot be free from fear. Others may take me to be a threat to them and so I could easily end up the victim of a pre-emptive strike. From these assumptions of equality, scarcity, and uncertainty, it follows, thinks Hobbes, that the state of nature will be a state of war: