Another school of philosophy grew out of Madhyamika. It was known as the Buddha-Nature (tathagatagarbha) school.
In this school, thinkers concluded that since nirvana and samsara cannot ultimately be separated (because of the artificial construction of opposites in human thought), then nirvana must also be present in all reality.
Thus all beings already have some element of nirvana and therefore the potential for full enlightenment.
This evolved into the Hua-Yen school of Buddhism in China, which in turn influenced the development of Zen Buddhism, with its emphasis on sudden, unpremeditated enlightenment (already inherent but unseen in the individual).
The inherent potential for nirvana was in other words the inner Buddha Nature found in each person.