If you were to ask people familiar with Buddhism to identify its two most important wisdom teachings, they’d probably say emptiness and the four noble truths.
If you were to ask them further which of the two teachings was more fundamental, they might hesitate, but most of them would probably put emptiness first, on the grounds that the four noble truths deal with a mental problem, while emptiness describes the way things in general are.
It wasn’t always this way. The Buddha himself gave more importance to the four noble truths, and it’s important to understand why.
When he boiled his teaching down to its shortest formulation, he said that he taught just dukkha—suffering and stress—and the cessation of dukkha (MN 22; SN 22:86).
The four noble truths expand on this formulation, defining what suffering is—clinging; how it’s caused—craving and ignorance; the fact that it can be brought to an end by abandoning its cause; and the path of practice that leads to that end.
Because part of the path of practice contains desire—the desire, in right effort, to act skillfully so as to go beyond suffering—the four noble truths also expand on one of the Buddha’s main observations about the phenomena of experience: that with the exception of nibbāna, they’re all rooted in desire (AN 10:58).
People aren’t simply passive recipients of their experience. Starting from their desires, they play an active role in shaping it. The strategy implied by the four noble truths is that desire should be retrained so that, instead of causing suffering, it helps act toward suffering’s end.
As for emptiness, the Buddha mentioned it only rarely, but one of his definitions for emptiness (SN 35:85) closely relates it to another teaching that he mentioned a great deal. That’s the teaching popularly known as the three characteristics, and that the Buddha himself called, not “characteristics,” but “perceptions”: the perception of inconstancy, the perception of suffering/stress, and the perception of not-self.
When explaining these perceptions, he taught that if you perceive fabricated things—all things conditioned by acts of intention—as inconstant, you’ll also see that they’re stressful and thus not worthy identifying as you or yours.
His purpose in teaching these perceptions was for them to be applied to suffering and its cause as a way of fostering dispassion for the objects of clinging and craving and for the acts of clinging and craving themselves. In this way, these perceptions were aids in carrying out the duties appropriate to the four noble truths: to comprehend suffering, to abandon its cause, to realize its cessation by developing the path.
In other words, the four noble truths and their duties supplied the context for the three perceptions and determined their role in the practice.
This reflection by Ajaan Geoff is from the book, First Things First, (pdf) pp. 20-21.