To call something “a fundamental principle of Buddhism” is correct only if, first, it is a principle that aims at the quenching of dukkha (suffering) and, second, it has a logic that one can see for oneself without having to believe others.
The Buddha refused to deal with those things that don’t lead to the extinction of dukkha. He didn’t discuss them. Take the question of whether or not there is rebirth after death. What is reborn? How is it reborn? What is its “karmic inheritance”? These questions don’t aim at the extinction of dukkha. That being so, they are not the Buddha’s teaching nor are they connected with it. They don’t lie within the range of Buddhism. Also, the one who asks about such matters has no choice but to believe indiscriminately any answer that’s given, because the one who answers won’t be able to produce any proofs and will just be speaking according to his own memory and feeling. The listener can’t see for himself and consequently must blindly believe the other’s words. Little by little the subject strays from dharma until it becomes something else altogether, unconnected with the extinction of dukkha.
Now, if we don’t raise those sorts of issues, we can ask instead, “Is there dukkha?” and “How can dukkha be extinguished?” The Buddha agreed to answer these questions. The listener can recognize the truth of every word of the answers without having to believe them blindly and can see the truth more and more clearly until he understands for himself.
There aren’t that many fundamental, or root, principles of dharma. The Buddha said that his teaching is “a single handful.” A passage in the Samyutta-nikaya makes that clear. While walking through the forest, the Buddha picked up a handful of fallen leaves and asked the monks who were present to decide which was the greater amount, the leaves in his hand or all the leaves in the forest. Of course, they all said that there were more leaves in the forest, that the difference was beyond comparison. Try to imagine the truth of this scene; clearly see how huge the difference is. The Buddha then said that, similarly, those things that he had realized were a great amount, equal to all the leaves in the forest. However, that which was necessary to know, those things that should be taught and practiced, were equal to the number of leaves in his hand.
From this it can be seen that, compared to all the myriad things in the world, the root principles to be practiced for the complete extinction of dukkha amount to a single handful. We must appreciate that this single handful is not a huge amount; it’s not something beyond our capabilities to reach and understand. This is the first important point that we must grasp if we want to lay the foundation for a correct understanding of the Buddha’s teachings.
The saying of the Buddha that deals with the practice regarding shunyata (voidness) is the saying that is the heart of Buddhism. It requires our careful attention. "Nothing whatsoever should be clung to as 'I' or 'mine.'" (Sabbe dhamma nalam abhinivesaya.) The Buddha himself declared that this is the summation of all the Tathagata’s [Buddha’s] teaching. He said that to have heard the phrase “Sabbe dhamma nalam abhinivesaya” is to have heard everything; to have put it into practice is to have practiced everything; and to have reaped its fruits is to have reaped every fruit. So we need not be afraid that there is too much to understand. When the Buddha compared the things that he had realized, which were as many as all the leaves in the forest, with those that he taught his followers to practice, which were a single handful, the single handful he referred to was just this principle of not grasping at or clinging to anything as being self or as belonging to self.
“To hear this phrase is to hear everything,” because all subjects are contained within it. Of all the things the Buddha taught, there wasn’t one that didn’t deal with dukkha and the elimination of dukkha. Grasping and clinging is the cause of dukkha. When there is grasping and clinging, that is dukkha. When there is no grasping and clinging—that is, being void of grasping and clinging—there is no dukkha. The practice is to make the non-arising of grasping and clinging absolute, final, and eternally void, so that no grasping and clinging can ever return. Just that is enough. There is nothing else to do.