The idea is that the contract is implicit. If you are living in society, partaking of its benefits, and not rebelling against the state, you have implicitly agreed to live by the rules of the state.
Agree is not the same as resign. Even if the society have no benefits for you, you cannot go away from it easily.
I never said it was easy. Yes, when you are a child you are not well informed enough to rebel against the state. But there are people who reject the social contract and have sought to over throw the government. They just happen to be rare, because once you are a mature adult, most people decide they prefer the rule of law. Even if they don't like all of the laws.
(In no way is my post advocating that people should go out and bomb Wall Street.)
presocratics?
Plato (and followers)
aristotle (and followers)
medieval cristianism
Hegel
Marx
Kant
Scropenhauer
husserl
heideger
Presocratics : Difficult to say, as we don't have much left.
Plato (and followers) : Plato revised his own theory at least twice. When thinking about Plato, most people assume his ontological theory is the one presented in the Republic. It's not. Plato took into account criticisms and reworked his theory. What is presented in his late dialogs is still considered to be the foundation of modern semantics, as his theory of ideas and concept is tweaked so it can explain the emergence of meaning based upon the relations between ideas.
Aristotle (and followers) : Aristotle never wrote a system. He used a set of logical tools to conduct inquiries about different parts of reality. His philosophy was first an empirical one. Trying to present Aristotle as a system, or to work out an aristotelian system was done only in the late middle ages, by christian thinkers like Aquinas, who overstated the importance of small parts of his writing (like the first cause.)
medieval cristianism : Is so diverse and full of different ideas you will have to be more precise. Ockham? Aquinas? Duns Scott? Avicenna A lot of good logical principles and ontological clarity we inherit from those authors. Seriously, read about the history of logic - the medieval islamic school was very important.
Hegel : Hegel =/= Hegel so Hegel++
Marx : I think we sadly read him because the posterity his political solution to economical problems had. That being said, I'm not an economist, but one of my friend is one and is happy to say that economists are finally starting to read him again as an economist, and not as a political thinker, and that you may find a lot of value in his economical though once you get past the political solutions. I don't know much more.
Kant : His systematic exploration of what you could expect from naive realism, why it was wrong and how it could be mended by critical realism is still a must read. He did show a lot about the logical assumptions under which we construct theories about reality and even when he was wrong, there is a lot to learn from his undertaking. He also re-wrote a good part of the first critique to take into accounts criticisms, and went to write two more books to "patched" his theory in many ways. Even at the end of his life, he still tried to correct his system when discovering a new difficulty it could not account for.
Schopenhauer : I don't know about his philosophy well enough to appraise it.
Husserl : He revised his theory every day of his life, writing a new book every month or so to refine his though. There is some really good stuff, and really bad stuff in Husserl, but he may be the most unsystematic philosopher out there, and one that was always in state of reworking his claims to take into account new problems/data.
Heidegger : Well. I too, want a cabin in the wood. Seriously, again, I find little I like in Heidegger, but if you take him for what he was, a counselor, you may find something if you are in an existential crisis. Heidegger is a moral philosopher. Do not look there for rational thinking about science.
Now, it's House on TV. So I must stop writing.