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.
The 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 to demonstrate 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. Material
So, we need to start by defining the word “spiritual.” It is easy. “Spiritual” simply means “nonmaterial.” Whether nonmaterial entities exist is another matter, but, by definition, anything not material 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 material things in the universe and its complement, the subset of all nonmaterial things in the universe. Thus, if we depict the universal set as a square where all material entities (elements) in it are contained in the circle inside the square, then all entities (elements) outside the circle are, by definition, nonmaterial :
We can argue whether there are any nonmaterial things in the universe—that is, if the set of all nonmaterial (“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 would admit that we have no proof that the set of nonmaterial entities is an empty set—that no nonmaterial entities exist in the universe. Thus, it behooves us to consider the possibility that spiritual entities might exist. As Shakespeare famously said:
There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.William Shakespeare, Hamlet
Come to think of it, information disembodied from its carrier is clearly nonmaterial. So, we already have one familiar example of a spiritual (that is nonmaterial) entity.
Our definition of spiritual as nonmaterial would not be very helpful if we did not define what is “material.” Material objects exist in three-dimensional physical spaces.
Descartes called material objects res extensa—things of substance that have spatial extensions. Although Descartes thought that these are the things we can see and touch, today we know forms of matter that we can neither see nor touch. Material fields, such as a gravitational field or an electromagnetic field, cannot be seen or touched. They also do not have defined spatial extensions like solid objects. Nevertheless, these fields are forms of matter. Moreover, from the standpoint of quantum field theory, all elementary particles are excitations of quantum fields. Thus, all material objects ultimately are made from fields. We classify fields as material substances, because they are defined in space. From here it follows that nonmaterial 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.” Descartes called nonmaterial things res cogitans—the stuff of the mind.
However, nonmaterial things include much more than ideas or products of mental activity. There are spiritual things that are beyond the grasp of the human mind. For this reason, we define spiritual things much more broadly—as the group of all nonmaterial substances.
The difficulty in proving (or disproving) the existence of spiritual things, is that nonmaterial things, by and large, do not couple (that is, interact) with material things. This should come as no surprise because, in the material 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, is trying to weigh the soul or detect it by other material means. Spiritual—nonmaterial—things usually do not couple or interact with material things, at least not in the way that material things, such as particles and fields, interact with each other. By way of example, hypothetical dark matter does not couple with ordinary matter in any other way besides exerting its gravitational pull. Otherwise, it is invisible and intangible.
Of course, if spiritual things do not interact with material things in any way at all, why do they matter? If the soul does not interact with the body, why should we care about the soul? But if the lack of interaction between the material and the immaterial were always the case, we would have no awareness of the soul’s existence. Similar to dark matter, the nonmaterial (spiritual) substance does not couple with any other matter except live matter (live organisms), which also presume to have some spiritual aspect to them. That aspect is called the “soul.” A soul interacts with the live body (of the human or other organism) it “inhabits.” How this coupling (interaction between material and spiritual) works is one of the greatest mysteries that needs to be solved. We need to explore how spiritual things and material things relate to each other, in this case, 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 G‑d, 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,” Goodreads, 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 material elements is Up, its complement Uc is the subset of all nonmaterial 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 matter. It could not be a part of the standard model and thus could not be detected in physical experiments, because it is not material 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 weighing dying patients just before and after death. He “determined” that the soul weighed 21 grams. MacDougall used a very small group of subjects—only four—and he dismissed inconsistent results. It is not surprising that these experiments could not be repeated and lacked merit. See MacDougall, “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 October 10, 2021.