The three marks of existence are Buddhism’s basic description of reality. These three simple truths, which characterize all things, are surprisingly transformative. They are:
Impermanence (Pali: annica): This truth is the foundation of Buddhism. The Buddha said that all compounded phenomena disintegrate. All things are made of parts, and all things fall apart. Another, blunter, way to put it is that everything dies. All of samsara is an attempt to deny this reality.
Suffering (dukkha): Every experience is marked by some quality of suffering, whether it’s extreme pain or a background sense of unease. As long as we struggle to maintain a sense of solid self, our lives will be marked by stress and fear. Our struggle will always be unsuccessful because of…
Non-Self: (annata): There is no solid, separate, single self. We have no core. We are simply the product of multiple causes and conditions. Impermanence describes how things are; non-self describes what they are not. Or, as Thich Nhat Hanh puts it, impermanence is emptiness in terms of time; non-self is emptiness in terms of space.
To these three, some teachers add a fourth: nirvana. This describes the absolute state free of all dualism. It marks all things because relative phenomena are not separate from the complete peace of the absolute.