Madhyamaka

Madhyamaka means "the middle way." You may recall that Buddha himself called his way the middle way in his very first sermon. He meant, at that time, the middle way between the extremes of hedonistic pleasure and extreme asceticism. But he may also have referred to the middle way between the competing philosophies of eternalism and annihilationism -- the belief that the soul exists forever and that the soul is annihilated at death. Or between materialism and nihilism.... An Indian monk by the name of Nagarjuna took this idea and expanded on it to create the philosophy that would be known as Madhyamaka, in a book called the Mulamadhyamaka-karika, written about 150 ad.
Basically a treatise on logical argument, it concludes that nothing is absolute, everything is relative, nothing exists on its own, everything is interdependent. All systems, beginning with the idea that each thing is what it is and not something else (Aristotle's law of the excluded middle), wind up contradicting themselves. Rigorous logic, in other words, leads one away from all systems, and to the concept of shunyata.
Shunyata means emptiness. This doesn't mean that nothing exists. It means that nothing exists in and of itself, but only as a part of a universal web of being. This would become a central concept in all branches of Mahayana. Of course, it is actually a restatement of the central Buddhist concepts of anatman, anitya, and dukkha!