What is the origin of the teaching that the human soul is invisible and immortal?

The difficulty lies in the fact that the meanings popularly attached to the English word “soul” stem primarily, not from the Hebrew or Christian Greek Scriptures, but from ancient Greek philosophy, actually pagan religious thought. Greek philosopher Plato, for example, quotes Socrates as saying: “The soul, . . . if it departs pure, dragging with it nothing of the body, . . . goes away into that which is like itself, into the invisible, divine, immortal, and wise, and when it arrives there it is happy, freed from error and folly and fear . . . and all the other human ills, and . . . lives in truth through all after time with the gods.”​—Phaedo, 80, D, E; 81, A.

Of course, such sources, treating as they do primarily of classical Greek writings, include all the meanings that the pagan Greek philosophers gave to the word, including that of “departed spirit,” “the immaterial and immortal soul,” “the spirit of the universe,” and “the immaterial principle of movement and life.” Evidently because some of the pagan philosophers taught that the soul emerged from the body at death, the term psy·kheʹ was also applied to the “butterfly or moth,” which creatures go through a metamorphosis, changing from caterpillar to winged creature.​—Liddell and Scott’s Greek-English Lexicon, revised by H. Jones, 1968, pp. 2026, 2027; Donnegan’s New Greek and English Lexicon, 1836, p. 1404.

The ancient Greek writers applied psy·kheʹ in various ways and were not consistent, their personal and religious philosophies influencing their use of the term. Of Plato, to whose philosophy the common ideas about the English “soul” may be attributed (as is generally acknowledged), it is stated: “While he sometimes speaks of one of [the alleged] three parts of the soul, the ‘intelligible,’ as necessarily immortal, while the other two parts are mortal, he also speaks as if there were two souls in one body, one immortal and divine, the other mortal.”​—The Evangelical Quarterly, London, 1931, Vol. III, p. 121, “Thoughts on the Tripartite Theory of Human Nature,” by A. McCaig.