In many religions, philosophical, and mythological traditions, the soul is the incorporeal essence of a living being. Depending on the philosophical system, a soul can either be mortal or immortal. In Judeo-Christianity, only human beings have immortal souls (although immortality is disputed within Judaism and may have been influenced by Plato). For example, the Catholic theologian Thomas Aquinas attributed "soul" () to all organisms but argued that only human souls are immortal. Other religions (most notably Hinduism and Jainism) hold that all biological organisms have souls, as did Aristotle, while some teach that even non-biological entities (such as rivers and mountains) possess souls. The latter belief is called animism.
Greek philosophers, such as Socrates, Plato and Aristotle, understood that the soul (ψυχή ) must have a logical faculty, the exercise of which was the most divine of human actions. At his defense trial, Socrates even summarized his teaching as nothing other than an exhortation for his fellow Athenians to excel in matters of the psyche since all bodily goods are dependent on such excellence (Apology 30a–b).
Anima mundi is the concept of a "world soul" connecting all living organisms on planet Earth.
The Modern English word "", derived from Old English sáwol, sáwel, was first attested in the 8th-century poem Beowulf v. 2820 and in the Vespasian Psalter 77.50. It is cognate with other German and Baltic terms for the same idea, including Gothic saiwala, Old High German sêula, sêla, Old Saxon sêola, Old Low Franconian sêla, sîla, Old Norse sála and Lithuanian siela. Further etymology of the Germanic word is uncertain. The original concept is meant to be 'coming from or belonging to the sea/lake', because of the German belief in souls being born out of and returning to sacred lakes, Old Saxon sêola (soul) compared to Old Saxon sêo (sea).