Guṇa depending on the context means "string, thread, or strand", or "virtue, merit, excellence", or "quality, peculiarity, attribute, property". The concept is originally notable as a feature of Samkhya philosophy, though possibly a later feature of it. The gunas are now a key concept in nearly all schools of Hindu philosophy. There are three gunas, according to this worldview, that have always been and continue to be present in all things and beings in the world. These three gunas are called: sattva (goodness, constructive, harmonious), rajas (passion, active, confused), and tamas (darkness, destructive, chaotic). All of these three gunas are present in everyone and everything, it is the proportion that is different, according to Hindu worldview. The interplay of these gunas defines the character of someone or something, of nature and determines the progress of life. In some contexts, it may mean "a subdivision, species, kind, quality", or an operational principle or tendency of something or someone. In human behavior studies, Guna means personality, innate nature and psychological attributes of an individual. Like all Sanskrit technical terms, guṇa can be difficult to summarize in a single word. Its original and common meaning is a thread, implying the original materials that weave together to make up reality. The usual, but approximate translation in common usage is "quality".