The nondual nature of reality is also part of the four noble truths. Although there are four truths, each truth contains the others; they can’t be considered completely separately from each other. If you fully understand one noble truth, you understand all four. If you really begin to understand suffering, you are already beginning to understand the path to its cessation. The four truths inter-are. Each one contains the others.
The first noble truth is ill-being. The second noble truth is the causes of ill-being, the thoughts and actions that put us on the path leading to ill-being. The third noble truth is well-being, the cessation of ill-being. The fourth noble truth is the path leading to well-being, the noble eightfold path.
The second noble truth is the action that leads to suffering, and the fourth noble truth is the action that leads to well-being, so in a sense they are two pairs of cause and effect. The second noble truth (the path of ill-being) leads us to the first (ill-being), and the fourth noble truth (the noble eightfold path) leads us to the third (well-being, the cessation of ill-being). Either we are walking the noble path or we are on the ignoble path that brings suffering to ourselves and others. We are always on one path or the other.
-Thich Nhat Hanh, “Good Citizens: Creating Enlightened Society”.