Locke and Hobbes agreed that a social contract is necessary to have anything resembling a civil society. Hobbes believed that human beings are not fundamentally different from non-human animals in terms of their motivations. This is one way to see why the social contract is necessary. Before human evolved there were societies proper because lower animals are not able to reason sufficiently to establish anything like a social contract. Hobbes is an egoist.
There are two forms of egoism, psychological egoism and ethical egoism. The first holds that people necessarily act out of self-interest. Ethical egoism is the view that it is morally right or good to act out of self-interest. Hobbes certainly accepts psychological egoism. This, too, can explain why he thinks that a social contract is necessary. For a group of people acting exclusively out of self-interest would simply try to exploit each other and the result would be something quite chaotic. A social contract is necessary, for Hobbes, because only by surrendering ourselves to the will of the sovereign can we escape the chaos that is the state of nature.
While Locke too believes in the necessity of a social contract his view is not as pessimistic about human nature as is the view of Hobbes. For Locke there is a crucial difference between people and other animals that is not exhausted by the fact that the former can reason—and thus are capable of forming a social contract—while the latter are not. Locke allowed motives other than self-interest and so it appears to be the case that he did not accept psychological egoism. However, Locke still believes that a kind of contract is necessary to safeguard property, which would be insecure in the absence of conventions such as would be established by a social contract.
For Hobbes, people have desires that are incompatible with the conditions of a state of nature, in which society (if there could be society at all) would be either complete chaos or a situation in which the stronger simply preyed on the weak. Thus, Hobbes held that people wan to avoid pain and to achieve pleasure, and they also have a desire for personal security. Notice that these desires are not inconsistent with Hobbes’s psychological egoism because they are all self-regarding in an obvious way. As suggested earlier, Hobbes believes that the best, and possibly the only, form of government is one in which there is a single ruler, or sovereign. People are willing to submit their wills and conducts to this sovereign because so doing is necessary to fulfill their desires for life, pleasure, and security. This leader, or perhaps it is better to call him or her a dictator, has complete freedom to do what is deemed necessary to keep the social contract in force.
Locke had a very different conception of the social contract and of the form that government should take. The state of nature is not fundamentally chaotic for Locke, as it is for Hobbes. But Locke nevertheless thinks that property will not be fully secure in the absence of a social contract that in effect lifts us out of the state of nature. It is from Locke that we get the famous idea that people should be free to act as they wish except insofar as their actions impinge on the freedoms of others, including the freedom to own property. Rather than being subject to the caprice of a sovereign, Locke believed that the social contract should include laws (and not merely civil laws) and impartial judges to adjudicate disputes.
Marx seems to have held that social contracts are constitutively connected with a capitalist economic system. Such a system involves privately owned goods, or private property, as well as services such as labor that are sold on the free market. There are several fundamental contradictions with capitalism, so understood. It is important to notice that Marx does not mean by “contradiction” a logical contradiction such as that a single proposition is both true and false. Contradictions for him are tensions that cause conflict and instability. There may not be anything inherently unjust in the very idea of private property for Marx, but he certainly thinks that any actual capitalist system is going to be unjust.
We can see why this is simply by considered the structure of private property and how it is employed in capitalism. I have labor to sell. Suppose that I am hired to work in a factory. If the owner of the factory is going to make a profit, I must produce more than I am compensated for. But then I am not really being paid what my labor is worth. This is why Marx, and many others, believed that wage labor was not importantly different from slavery. Furthermore, as the method of production becomes more efficient and streamlined it is inevitable that I will become alienated from the results of my labor. If I work on a car assembly line, for example, I may never see what a completed car looks like. I simply produce the same component for hours every day, day after day. Marx seems to suggest that it is unnatural to expect people to work hard if they are deprived of the sense of satisfaction or pride that can only come with knowing that one’s labor has been put to good use. Eventually alienation and other sources of dissatisfaction, according to Marx, will lead the oppressed proletariat to rise up and rebel against the owners of the means of production. So capitalism will inevitably lead to a revolution which supplants it.