Most Americans believe in an immortal soul. Most scientists think the soul does not exist. Why do so many people believe in an immortal soul? Why do scientists not believe in it? What is a soul, anyway? Do we need a soul? We will explore these and related questions in this essay.
French philosopher and mathematician René Descartes famously said “If philosophers were always in agreement about the meaning of words, almost all their disputes would evaporate.” There is hardly a better example of this truism than the concept of a soul—one of the most misunderstood and ill-defined concepts. A soul is perceived as spiritual, spooky, and otherworldly. While there is nothing spooky or otherworldly about a soul, it is indeed a spiritual concept. However, that knowledge does not help us, so long as we perceive the word “spiritual” to be as ill-defined, spooky, and otherworldly as the soul itself.
Spiritual vs. Physical
So, we need to start by defining the word “spiritual.” It is easy. “Spiritual” simply means “nonphysical.” Whether nonphysical entities exist is another matter, but by definition, anything not physical is called “spiritual.”
If we were to consider a universal set U of everything that exists in the universe, this set could be divided into two complementary subsets: the subset of all physical things in the universe and its complement, the subset of all nonphysical things in the universe. Thus, if we depict the universal set as a square where all physical entities (elements) in it are contained in the circle inside the square, then all entities (elements) outside the circle are, by definition, nonphysical:
We can argue whether there are any nonphysical things in the universe—that is, if the set of all nonphysical (“spiritual”) things is an empty set, but at least we have a rigorous and unambiguous definition of “spiritual” entities that prevents the conversation from descending into muddy New Age psychobabble. Most scientists or philosophers will admit that we have no proof that the set of nonphysical entities is an empty set—that no nonphysical entities exist in the universe. Thus, it behooves us to consider the possibility that such spiritual entities might exist. Come to think of it, information disembodied from its carrier is clearly nonphysical. So, we already have one familiar example of a spiritual (that is nonphysical) entity.
Our definition of spiritual as nonphysical would not be very helpful if we did not define what is physical. Physical objects exist in three-dimensional physical spaces. From here it follows that nonphysical entities exist outside of our three-dimensional physical space. They also exist in a space, but a different kind of space. Spiritual entities exist in abstract conceptual spaces, where proximity is measured by the degree of similarity. We discussed abstract conceptual spaces in my earlier essay, “Physics of Tzimtzum II — Collapse of the Wave Function.” We will have more to say about this later.
The difficulty in proving (or disproving) the existence of spiritual things is that nonphysical things, by and large, do not couple with (that is, do not interact with) physical things. This should come as no surprise because, in the physical world, we have a similar phenomenon. To detect an electromagnetic field, we need to use a charged particle, such as an electron, that couples with an electromagnetic field. Trying to use a neutral particle to detect an electromagnetic field is a fool’s errand. So, too, trying to weigh the soul or detect it by other physical means is a fool’s errand. Spiritual—nonphysical—things usually do not couple (interact) with physical things, at least not in the way that physical things, such as particles and fields, interact with each other.
Of course, if spiritual things do not interact with physical things in any way at all, why would they matter? If the soul did not interact with the body, why would we care about the soul? If that were the case, we would not have been aware of the soul’s existence. Actually, a soul does interact with the live body (human or any other organism) it “inhabits.” How this works is one of the greatest mysteries that needs to be solved. We need to explore how spiritual things and physical things relate to each other, how the soul relates to, and interacts with, the body. We shall explore this in subsequent installments. But first, we should explore what Judaism has to say about the concept of the soul. Please God, we will do this in the next installment.
 See “Do You Believe in Survival of the Soul After Death?,” Statista, https://www.statista.com/statistics/632117/united-states-belief-in-survival-of-the-soul-after-death/ (retrieved October 5, 2021).
 René Descartes, Rules for the Direction of the Mind, see online https://www.goodreads.com/author/quotes/36556.Ren_Descartes?page=1 (retrieved October 22, 2021).
 In set theory, the complement of a set A, denoted as Ac, is the set of all elements not in A. When all elements of A and Ac are elements of a set U, the absolute complement of A is the set of elements in U that are not in A. Thus, if a subset of U that has all physical elements is Up, its complement Uc is the subset of all nonphysical elements.
 Using the notations of set theory, Uc = Ø.
 Sean Carroll wrote that the idea of the soul is incompatible with quantum field theory. He writes, “Not only is new physics required, but dramatically new physics. Within QFT, there can’t be a new collection of ‘spirit particles’ and ‘spirit forces’ that interact with our regular atoms, because we would have detected them in existing experiments.” See Sean M. Carroll, “Physics and the Immortality of the Soul,” Scientific American, May 23, 2011, retrieved October 10, 2021. This argument misses the point—the soul does not interact with ordinary physical matter. It could not be a part of the standard model and thus could not [IS THIS WHAT YOU MEAN, could NOT?] be detected in physical experiments, because it is not physical and, as such, it does not obey the laws of physics.
 Rabbi Aryeh Kaplan, “Inner Space: Introduction to Kabbalah, Meditation and Prophecy,” ed. Abraham Sutton (Jerusalem: Moznaim, 1990), pp. 19, 191 (note 33) (see further sources there).
 In 1901, the physician Duncan MacDougall attempted to weigh the human soul by weight dying patients just before and after that patient expired. He “determined” that the soul weighed 21 grams. MacDougall used a very small group of subjects—only four—and he dismissed the inconsistent results. It is not surprising that these experiments could not be repeated and lack any merit. See MacDougall, Duncan, “The Soul: Hypothesis Concerning Soul Substance Together with Experimental Evidence of the Existence of Such Substance,” American Medicine, New Series, 1907, 2: 240–43. See also Benjamin Radford, “How Much Does the Soul Weigh?” Live Science, December 01, 2012. (https://www.livescience.com/32327-how-much-does-the-soul-weigh.html, retrieved on October 10, 2021.) Originally published on QuantumTorah.com on 10/22/2012.