Kleshas (Sanskrit, also kleśa; Pali: kilesa; Tibetan: nyon mongs), in Buddhism, are mental states that cloud the mind and manifest in unwholesome actions. Kleshas include states of mind such as anxiety, fear, anger, jealousy, desire, depression, etc. Contemporary translators use a variety of English words to translate the term kleshas, such as: afflictions, defilements, destructive emotions, disturbing emotions, negative emotions, mind poisons, etc.

In the contemporary Mahayana and Theravada Buddhist traditions, the three kleshas of ignorance, attachment, and aversion are identified as the root or source of all other kleshas. These are referred to as the three poisons in the Mahayana tradition, or as the three unwholesome roots in the Theravada tradition.

While the early Buddhist texts of the Pali canon do not specifically enumerate the three root kleshas, over time the three poisons (and the kleshas generally) came to be seen as the very roots of samsaric existence.

In the Pali Canon's discourses (sutta), kilesa is often associated with the various passions that defile bodily and mental states. In the Pali Canon's Abhidhamma and post-canonical Pali literature, ten defilements are identified, the first three of which – greed, hate, delusion – are considered to be the "roots" of suffering.

Buddhism is a religion and philosophy indigenous to the Indian subcontinent and encompasses a variety of traditions, beliefs, and practices largely based on teachings attributed to Siddhartha Gautama, who is commonly known as the Buddha (meaning "the awakened one" in Sanskrit and Pāli). The Buddha lived and taught in the eastern part of Indian subcontinent some time between the 6th and 4th centuries BCE. He is recognized by Buddhists as an awakened or enlightened teacher who shared his insights to help sentient beings end ignorance (avidyā), craving (taṇhā), and suffering (dukkha), by recognizing dependent origination and sunyata, and attain Nirvana.

Two major branches of Buddhism are recognized: Theravada ("The School of the Elders") and Mahayana ("The Great Vehicle"). Theravada has a widespread following in Sri Lanka and Southeast Asia. Mahayana is found throughout East Asia and includes the traditions of Pure Land, Zen, Nichiren Buddhism, Tibetan Buddhism, Shingon, Tiantai (Tendai) and Shinnyo-en. In some classifications, Vajrayana—practiced mainly in Tibet and Mongolia, and adjacent parts of China and Russia—is recognized as a third branch, while others classify it as a part of Mahayana. There are other categorisations of these three Vehicles or Yanas.

Jan Westerhoff is a philosopher and orientalist with specific interests in metaphysics and the philosophy of language. He was educated at Trinity College, Cambridge and SOAS. At present he is a Reader in Philosophy at the University of Durham as well Research Associate at SOAS. For two years he was Research Fellow in Philosophy at the City University of New York and seminar associate at Columbia University. Before that he was a Junior Lecturer in philosophy at the University of Oxford.

He is a specialist in metaphysics and Indo-Tibetan philosophy. His research interests also include the history of ideas in the sixteenth and seventeenth centuries.