Soul. What a beautiful and nettlesome word. I've used the word twice in the titles of books: The Soul of the Night: An Astronomical Pilgrimage, and The Virgin and the Mousetrap: Essays in Search of the Soul of Science. It seems the prefect word to describe the elusive essence of a thing, the thing that is too broad and subtle to be captured by a formula or theorem. The thing that is always the object of a search or a pilgrimage. Once you have found it, it is no longer soul.
Soul is the source of our joy and our anxiety. Joy because it beckons us forward; anxiety because it ever recedes before our grasp. Anxiety too because of the dualistic burden of the word. "The immaterial essence, animating principle, or actuating cause of an individual life, usually thought to be immortal," the dictionary says in its first definition of the word. Immaterial! Immortal! The ghost in the machine. The sprite that will fly free when bones and sinews turn to dust.
We know how Walt Whitman regarded the soul, how he spelled it out in his poem I Sing the Body Electric. "...Head, neck, hair, ears, drop and tympan of the ears,/ Eyes, eye-fringes, iris of the eye, eyebrows, and the waking and sleeping of the lids,/ mouth, tongue, lips, teeth, roof of the mouth, jaws, and the jaw hinges/...The lung-sponges, the stomach-sac, the bowels sweet and clean/...The womb, the teats, nipples, breast-milk, tears, laughter, weeping, love-looks, love-perturbations and risings/...The thin red jellies within you or within me, the bones and the marrow in the bones...etc., etc." -- to merely dip into his exuberant praisings of the flesh. "Oh I say now these are the soul!" he enthuses.
Yes. It is enough. One might spend a lifetime with a person -- with that long catalog of what Whitman calls the "parts and poems of the body" -- and still not know her soul. Material? Why not? What a thing is matter! In the formulations of contemporary physics, matter is all vibration, resonance, and inexhaustible potentialities. A literal music of the cosmos. Who in such a world needs the so-called immaterial, the immortal? Who needs a ghost in the machine when the machine itself is a thing of such infinite surprise?