Soul

The souls of Pe and Nekhen towing the royal bargue on a relief of Ramesses II's temple in Abydos.

In the ancient Egyptian religion, an individual was believed to be made up of various elements, some physical and some spiritual. Similar ideas are found in ancient Assyrian and Babylonian religion. The Kuttamuwa stele, a funeral stele for an 8th-century BCE royal official from Sam'al, describes Kuttamuwa requesting that his mourners commemorate his life and his afterlife with feasts "for my soul that is in this stele". It is one of the earliest references to a soul as a separate entity from the body. The 800-pound (360 kg) basalt stele is 3 ft (0.91 m) tall and 2 ft (0.61 m) wide. It was uncovered in the third season of excavations by the Neubauer Expedition of the Oriental Institute in Chicago, Illinois.[6]

The Baháʼí Faith affirms that "the soul is a sign of God, a heavenly gem whose reality the most learned of men hath failed to grasp, and whose mystery no mind, however acute, can ever hope to unravel".[7] Bahá'u'lláh stated that the soul not only continues to live after the physical death of the human body, but is, in fact, immortal.[8] Heaven can be seen partly as the soul's state of nearness to God; and hell as a state of remoteness from God. Each state follows as a natural consequence of individual efforts, or the lack thereof, to develop spiritually.[9] Bahá'u'lláh taught that individuals have no existence prior to their life here on earth and the soul's evolution is always towards God and away from the material world.[9]

Depiction of a soul being carried to heaven by two angels by William Bouguereau

According to a common Christian eschatology, when people die, their souls will be judged by God and determined to go to Heaven or to Hades awaiting the resurrection. Other Christians understand the soul as the life, and believe that the dead have no life until after the resurrection (Christian conditionalism). Some Christians believe that the souls and bodies of the unrighteous will be destroyed in Hell rather than suffering eternally (annihilationism). Believers will inherit eternal life either in Heaven, or in a Kingdom of God on earth, and enjoy eternal fellowship with God.

The Damned Soul. Drawing by Michelangelo Buonarroti c. 1525

The "origin of the soul" has provided a vexing question in Christianity. The major theories put forward include soul creationism, traducianism, and pre-existence. According to soul creationism, God creates each individual soul directly, either at the moment of conception or some later time. According to traducianism, the soul comes from the parents by natural generation. According to the preexistence theory, the soul exists before the moment of conception. There have been differing thoughts regarding whether human embryos have souls from conception, or whether there is a point between conception and birth where the fetus acquires a soul, consciousness, and/or personhood. Stances in this question might play a role in judgements on the morality of abortion.[10][11][12]

Augustine (354-430), one of western Christianity's most influential early Christian thinkers, described the soul as "a special substance, endowed with reason, adapted to rule the body". Some Christians espouse a trichotomic view of humans, which characterizes humans as consisting of a body (soma), soul (psyche), and spirit (pneuma).[13] However, the majority of modern Bible scholars point out how the concepts of "spirit" and of "soul" are used interchangeably in many biblical passages, and so hold to dichotomy: the view that each human comprises a body and a soul. Paul said that the "body wars against" the soul, "For the word of God is living and active and sharper than any two-edged sword, and piercing as far as the division of soul and spirit" (Heb 4:12 NASB), and that "I buffet my body", to keep it under control.

The present Catechism of the Catholic Church states that the term soul “refers to the innermost aspect of [persons], that which is of greatest value in [them], that by which [they are] most especially in God's image: ‘soul’ signifies the spiritual principle in [humanity]”.[14] All souls living and dead will be judged by Jesus Christ when he comes back to earth. The Catholic Church teaches that the existence of each individual soul is dependent wholly upon God: "The doctrine of the faith affirms that the spiritual and immortal soul is created immediately by God."[15]

Depiction of the soul on a 17th-century tombstone at the cemetery of the Old Dutch Church of Sleepy Hollow

Protestants generally believe in the soul's existence, but fall into two major camps about what this means in terms of an afterlife. Some, following Calvin,[16] believe in the immortality of the soul and conscious existence after death, while others, following Luther,[17] believe in the mortality of the soul and unconscious "sleep" until the resurrection of the dead.[18] Various new religious movements deriving from Adventism—including Christadelphians,[19] Seventh-day Adventists and Jehovah's Witnesses[20][21]—similarly believe that the dead do not possess a soul separate from the body and are unconscious until the resurrection.

The Church of Jesus Christ of Latter-day Saints teaches that the spirit and body together constitute the Soul of Man (Mankind). "The spirit and the body are the soul of man."[22] Latter-day Saints believe that the soul is the union of a pre-existing, God-made spirit[23][24][25] and a temporal body, which is formed by physical conception on earth. After death, the spirit continues to live and progress in the Spirit world until the resurrection, when it is reunited with the body that once housed it. This reuniting of body and spirit results in a perfect soul that is immortal and eternal and capable of receiving a fulness of joy.[26][27] Latter-day Saint cosmology also describes "intelligences" as the essence of consciousness or agency. These are co-eternal with God, and animate the spirits.[28] The union of a newly-created spirit body with an eternally-existing intelligence constitutes a "spirit birth" and justifies God's title "Father of our spirits".[29][30][31]

Some Confucian traditions contrast a spiritual soul with a corporeal soul.[32]

Ātman is a Sanskrit word that means inner self or soul.[33][34][35] In Hindu philosophy, especially in the Vedanta school of Hinduism, Ātman is the first principle,[36] the true self of an individual beyond identification with phenomena, the essence of an individual. In order to attain liberation (moksha), a human being must acquire self-knowledge (atma jnana), which is to realize that one's true self (Ātman) is identical with the transcendent self Brahman according to Advaita Vedanta.[34][37]

The six orthodox schools of Hinduism believe that there is Ātman (self, essence) in every being.[38]

In Hinduism and Jainism, a jiva (Sanskrit: जीव, jīva, alternative spelling jiwa; Hindi: जीव, jīv, alternative spelling jeev) is a living being, or any entity imbued with a life force.[39]

The concept of jiva in Jainism is similar to atman in Hinduism. However, some Hindu traditions differentiate between the two concepts, with jiva considered as individual self, while atman as that which is universal unchanging self that is present in all living beings and everything else as the metaphysical Brahman.[40][41][42] The latter is sometimes referred to as jiva-atman (a soul in a living body).[40] According to Brahma Kumaris, the soul is an eternal point of light.

The Quran, the holy book of Islam, uses two words to refer to the soul: rūḥ (translated as spirit, consciousness, pneuma or "soul") and nafs (translated as self, ego, psyche or "soul"),[43][44] cognates of the Hebrew nefesh and ruach. The two terms are frequently used interchangeably, though rūḥ is more often used to denote the divine spirit or "the breath of life", while nafs designates one's disposition or characteristics.[45] In Islamic philosophy, the immortal rūḥ "drives" the mortal nafs, which comprises temporal desires and perceptions necessary for living.[46] Two of the passages in the Quran that mention the rûh occur in chapters 17 ("The Night Journey") and 39 ("The Troops"):

And they ask you, [O Muhammad], about the Rûh. Say, "The Rûh is of the affair of my Lord. And mankind has not been given of knowledge except a little.

— Quran 17:85

Allah takes the souls at the time of their death, and those that do not die [He takes] during their sleep. Then He keeps those for which He has decreed death and releases the others for a specified term. Indeed in that are signs for a people who give thought..

— Quran 39:42

In Jainism, every living being, from plant or bacterium to human, has a soul and the concept forms the very basis of Jainism. According to Jainism, there is no beginning or end to the existence of soul. It is eternal in nature and changes its form until it attains liberation.

In Jainism, jiva is the immortal essence or soul of a living organism (human, animal, fish or plant etc.) which survives physical death.[47] The concept of Ajiva in Jainism means "not soul", and represents matter (including body), time, space, non-motion and motion.[47] In Jainism, a Jiva is either samsari (mundane, caught in cycle of rebirths) or mukta (liberated).[48][49]

According to this belief until the time the soul is liberated from the saṃsāra (cycle of repeated birth and death), it gets attached to one of these bodies based on the karma (actions) of the individual soul. Irrespective of which state the soul is in, it has got the same attributes and qualities. The difference between the liberated and non-liberated souls is that the qualities and attributes are manifested completely in case of siddha (liberated soul) as they have overcome all the karmic bondages whereas in case of non-liberated souls they are partially exhibited. Souls who rise victorious over wicked emotions while still remaining within physical bodies are referred to as arihants.[50]

Concerning the Jain view of the soul, Virchand Gandhi said

the soul lives its own life, not for the purpose of the body, but the body lives for the purpose of the soul. If we believe that the soul is to be controlled by the body then soul misses its power.[51]

The Hebrew terms נפש‎ nefesh (literally "living being"), רוח‎ ruach (literally "wind"), נשמה‎ neshamah (literally "breath"), חיה‎ chayah (literally "life") and יחידה‎ yechidah (literally "singularity") are used to describe the soul or spirit.[52]

In Judaism the soul was believed to be given by God to Adam as mentioned in Genesis,

Then the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.

— Genesis 2:7

Judaism relates the quality of one's soul to one's performance of the commandments (mitzvot) and reaching higher levels of understanding, and thus closeness to God. A person with such closeness is called a tzadik. Therefore, Judaism embraces the commemoration of the day of one's death, nahala/Yahrtzeit and not the birthday[53] as a festivity of remembrance, for only toward the end of life's struggles, tests and challenges could human souls be judged and credited for righteousness.[54][55] Judaism places great importance on the study of the souls.[56]

Kabbalah and other mystic traditions go into greater detail into the nature of the soul. Kabbalah separates the soul into five elements, corresponding to the five worlds:

1. Nefesh, related to natural instinct.
2. Ruach, related to emotion and morality.
3. Neshamah, related to intellect and the awareness of God.
4. Chayah, considered a part of God, as it were.
5. Yechidah. This aspect is essentially one with God.

Kabbalah also proposed a concept of reincarnation, the gilgul. (See also nefesh habehamit the "animal soul".)

The Scientology view is that a person does not have a soul, it is a soul. A person is immortal, and may be reincarnated if they wish. The Scientology term for the soul is "thetan", derived from the Greek word "theta", symbolizing thought. Scientology counselling (called auditing) addresses the soul to improve abilities, both worldly and spiritual.

The belief in soul dualism is found throughout most Austronesian shamanistic traditions. The reconstructed Proto-Austronesian word for the "body soul" is *nawa ("breath", "life", or "vital spirit"). It is located somewhere in the abdominal cavity, often in the liver or the heart (Proto-Austronesian *qaCay).[57][58] The "free soul" is located in the head. Its names are usually derived from Proto-Austronesian *qaNiCu ("ghost", "spirit [of the dead]"), which also apply to other non-human nature spirits. The "free soul" is also referred to in names that literally mean "twin" or "double", from Proto-Austronesian *duSa ("two").[59][60] A virtuous person is said to be one whose souls are in harmony with each other, while an evil person is one whose souls are in conflict.[61]

The "free soul" is said to leave the body and journey to the spirit world during sleep, trance-like states, delirium, insanity, and death. The duality is also seen in the healing traditions of Austronesian shamans, where illnesses are regarded as a "soul loss" and thus to heal the sick, one must "return" the "free soul" (which may have been stolen by an evil spirit or got lost in the spirit world) into the body. If the "free soul" can not be returned, the afflicted person dies or goes permanently insane.[62]

In some ethnic groups, there can also be more than two souls. Like among the Tagbanwa people, where a person is said to have six souls – the "free soul" (which is regarded as the "true" soul) and five secondary souls with various functions.[57]

Kalbo Inuit groups believe that a person has more than one type of soul. One is associated with respiration, the other can accompany the body as a shadow.[63] In some cases, it is connected to shamanistic beliefs among the various Inuit groups.[64] Also Caribou Inuit groups believed in several types of souls.[65]

The shaman heals within the spiritual dimension by returning 'lost' parts of the human soul from wherever they have gone. The shaman also cleanses excess negative energies, which confuse or pollute the soul.

Sikhism considers soul (atma) to be part of God (Waheguru). Various hymns are cited from the holy book Guru Granth Sahib (SGGS) that suggests this belief. "God is in the Soul and the Soul is in the God."[66] The same concept is repeated at various pages of the SGGS. For example: "The soul is divine; divine is the soul. Worship Him with love."[67] and "The soul is the Lord, and the Lord is the soul; contemplating the Shabad, the Lord is found."[68]

The atma or soul according to Sikhism is an entity or "spiritual spark" or "light" in our body because of which the body can sustain life. On the departure of this entity from the body, the body becomes lifeless – No amount of manipulations to the body can make the person make any physical actions. The soul is the 'driver' in the body. It is the roohu or spirit or atma, the presence of which makes the physical body alive.

Many religious and philosophical traditions support the view that the soul is the ethereal substance – a spirit; a non-material spark – particular to a unique living being. Such traditions often consider the soul both immortal and innately aware of its immortal nature, as well as the true basis for sentience in each living being. The concept of the soul has strong links with notions of an afterlife, but opinions may vary wildly even within a given religion as to what happens to the soul after death. Many within these religions and philosophies see the soul as immaterial, while others consider it possibly material.

According to Chinese traditions, every person has two types of soul called hun and po (魂 and 魄), which are respectively yang and yin. Taoism believes in ten souls, sanhunqipo (三魂七魄) "three hun and seven po".[69] A living being that loses any of them is said to have mental illness or unconsciousness, while a dead soul may reincarnate to a disability, lower desire realms, or may even be unable to reincarnate.

Charon (Greek) who guides dead souls to the Underworld. 4th century BCE.

In theological reference to the soul, the terms "life" and "death" are viewed as emphatically more definitive than the common concepts of "biological life" and "biological death". Because the soul is said to be transcendent of the material existence, and is said to have (potentially) eternal life, the death of the soul is likewise said to be an eternal death. Thus, in the concept of divine judgment, God is commonly said to have options with regard to the dispensation of souls, ranging from Heaven (i.e., angels) to hell (i.e., demons), with various concepts in between. Typically both Heaven and hell are said to be eternal, or at least far beyond a typical human concept of lifespan and time.

According to Louis Ginzberg, the soul of Adam is the image of God.[70] Every soul of human also escapes from the body every night, rises up to heaven, and fetches new life thence for the body of man.[71]

In Brahma Kumaris, human souls are believed to be incorporeal and eternal. God is considered to be the Supreme Soul, with maximum degrees of spiritual qualities, such as peace, love and purity.[72]

In Helena Blavatsky's Theosophy, the soul is the field of our psychological activity (thinking, emotions, memory, desires, will, and so on) as well as of the so-called paranormal or psychic phenomena (extrasensory perception, out-of-body experiences, etc.). However, the soul is not the highest, but a middle dimension of human beings. Higher than the soul is the spirit, which is considered to be the real self; the source of everything we call "good"—happiness, wisdom, love, compassion, harmony, peace, etc. While the spirit is eternal and incorruptible, the soul is not. The soul acts as a link between the material body and the spiritual self, and therefore shares some characteristics of both. The soul can be attracted either towards the spiritual or towards the material realm, being thus the "battlefield" of good and evil. It is only when the soul is attracted towards the spiritual and merges with the Self that it becomes eternal and divine.

Rudolf Steiner claimed classical trichotomic stages of soul development, which interpenetrated one another in consciousness:[73]

• The "sentient soul", centering on sensations, drives, and passions, with strong conative (will) and emotional components;
• The "intellectual" or "mind soul", internalizing and reflecting on outer experience, with strong affective (feeling) and cognitive (thinking) components; and
• The "consciousness soul", in search of universal, objective truths.

In Surat Shabda Yoga, the soul is considered to be an exact replica and spark of the Divine. The purpose of Surat Shabd Yoga is to realize one's True Self as soul (Self-Realisation), True Essence (Spirit-Realisation) and True Divinity (God-Realisation) while living in the physical body.

Similarly, the spiritual teacher Meher Baba held that "Atma, or the soul, is in reality identical with Paramatma the Oversoul – which is one, infinite, and eternal...[and] [t]he sole purpose of creation is for the soul to enjoy the infinite state of the Oversoul consciously."[74]

Eckankar, founded by Paul Twitchell in 1965, defines Soul as the true self; the inner, most sacred part of each person.[75]

G.I. Gurdjieff taught that humans are not born with immortal souls but could develop them through certain efforts.[76]

Plato (left) and Aristotle (right), a detail of The School of Athens, a fresco by Raphael.

Drawing on the words of his teacher Socrates, Plato considered the psyche to be the essence of a person, being that which decides how we behave. He considered this essence to be an incorporeal, eternal occupant of our being. Plato said that even after death, the soul exists and is able to think. He believed that as bodies die, the soul is continually reborn (metempsychosis) in subsequent bodies. However, Aristotle believed that only one part of the soul was immortal, namely the intellect (logos). The Platonic soul consists of three parts:[80]

1. the logos, or logistikon (mind, nous, or reason)
2. the thymos, or thumetikon (emotion, spiritedness, or masculine)
3. the eros, or epithumetikon (appetitive, desire, or feminine)

The parts are located in different regions of the body:

1. logos is located in the head, is related to reason and regulates the other part.
2. thymos is located near the chest region and is related to anger.
3. eros is located in the stomach and is related to one's desires.

Plato also compares the three parts of the soul or psyche to a societal caste system. According to Plato's theory, the three-part soul is essentially the same thing as a state's class system because, to function well, each part must contribute so that the whole functions well. Logos keeps the other functions of the soul regulated.

The structure of the souls of plants, animals, and humans, according to Aristotle, with Bios, Zoê, and Psūchê

Aristotle (384–322 BCE) defined the soul, or Psūchê (ψυχή), as the "first actuality" of a naturally organized body,[81] and argued against its separate existence from the physical body. In Aristotle's view, the primary activity, or full actualization, of a living thing constitutes its soul. For example, the full actualization of an eye, as an independent organism, is to see (its purpose or final cause).[82] Another example is that the full actualization of a human being would be living a fully functional human life in accordance with reason (which he considered to be a faculty unique to humanity).[83] For Aristotle, the soul is the organization of the form and matter of a natural being which allows it to strive for its full actualization. This organization between form and matter is necessary for any activity, or functionality, to be possible in a natural being. Using an artifact (non-natural being) as an example, a house is a building for human habituation, but for a house to be actualized requires the material (wood, nails, bricks, etc.) necessary for its actuality (i.e. being a fully functional house). However, this does not imply that a house has a soul. In regards to artifacts, the source of motion that is required for their full actualization is outside of themselves (for example, a builder builds a house). In natural beings, this source of motion is contained within the being itself.[84] Aristotle elaborates on this point when he addresses the faculties of the soul.

The various faculties of the soul, such as nutrition, movement (peculiar to animals), reason (peculiar to humans), sensation (special, common, and incidental) and so forth, when exercised, constitute the "second" actuality, or fulfillment, of the capacity to be alive. For example, someone who falls asleep, as opposed to someone who falls dead, can wake up and live their life, while the latter can no longer do so.

Aristotle identified three hierarchical levels of natural beings: plants, animals, and people, having three different degrees of soul: Bios (life), Zoë (animate life), and Psuchë (self-conscious life). For these groups, he identified three corresponding levels of soul, or biological activity: the nutritive activity of growth, sustenance and reproduction which all life shares (Bios); the self-willed motive activity and sensory faculties, which only animals and people have in common (Zoë); and finally "reason", of which people alone are capable (Pseuchë).

Aristotle's discussion of the soul is in his work, De Anima (On the Soul). Although mostly seen as opposing Plato in regard to the immortality of the soul, a controversy can be found in relation to the fifth chapter of the third book: in this text both interpretations can be argued for, soul as a whole can be deemed mortal, and a part called "active intellect" or "active mind" is immortal and eternal.[85] Advocates exist for both sides of the controversy, but it has been understood that there will be permanent disagreement about its final conclusions, as no other Aristotelian text contains this specific point, and this part of De Anima is obscure.[86] Further, Aristotle states that the soul helps humans find the truth, and understanding the true purpose or role of the soul is extremely difficult.[87]

Following Aristotle, Avicenna (Ibn Sina) and Ibn al-Nafis, an Arab physician, further elaborated upon the Aristotelian understanding of the soul and developed their own theories on the soul. They both made a distinction between the soul and the spirit, and the Avicennian doctrine on the nature of the soul was influential among the Scholastics. Some of Avicenna's views on the soul include the idea that the immortality of the soul is a consequence of its nature, and not a purpose for it to fulfill. In his theory of "The Ten Intellects", he viewed the human soul as the tenth and final intellect.[88][89]

While he was imprisoned, Avicenna wrote his famous "Floating man" thought experiment to demonstrate human self-awareness and the substantial nature of the soul.[90] He told his readers to imagine themselves suspended in the air, isolated from all sensations, which includes no sensory contact with even their own bodies. He argues that in this scenario one would still have self-consciousness. He thus concludes that the idea of the self is not logically dependent on any physical thing, and that the soul should not be seen in relative terms, but as a primary given, a substance. This argument was later refined and simplified by René Descartes in epistemic terms, when he stated: "I can abstract from the supposition of all external things, but not from the supposition of my own consciousness."[91]

Avicenna generally supported Aristotle's idea of the soul originating from the heart, whereas Ibn al-Nafis rejected this idea and instead argued that the soul "is related to the entirety and not to one or a few organs". He further criticized Aristotle's idea whereby every unique soul requires the existence of a unique source, in this case the heart. Al-Nafis concluded that "the soul is related primarily neither to the spirit nor to any organ, but rather to the entire matter whose temperament is prepared to receive that soul," and he defined the soul as nothing other than "what a human indicates by saying "I".[92]

Following Aristotle (whom he referred to as "the Philosopher") and Avicenna, Thomas Aquinas (1225–74) understood the soul to be the first actuality of the living body. Consequent to this, he distinguished three orders of life: plants, which feed and grow; animals, which add sensation to the operations of plants; and humans, which add intellect to the operations of animals.

Concerning the human soul, his epistemological theory required that, since the knower becomes what he knows, the soul is definitely not corporeal—if it is corporeal when it knows what some corporeal thing is, that thing would come to be within it.[93] Therefore, the soul has an operation which does not rely on a body organ, and therefore the soul can exist without a body. Furthermore, since the rational soul of human beings is a subsistent form and not something made of matter and form, it cannot be destroyed in any natural process.[94] The full argument for the immortality of the soul and Aquinas' elaboration of Aristotelian theory is found in Question 75 of the First Part of the Summa Theologica.

In his discussions of rational psychology, Immanuel Kant (1724–1804) identified the soul as the "I" in the strictest sense, and argued that the existence of inner experience can neither be proved nor disproved.

We cannot prove a priori the immateriality of the soul, but rather only so much: that all properties and actions of the soul cannot be recognized from materiality.

It is from the "I", or soul, that Kant proposes transcendental rationalization, but cautions that such rationalization can only determine the limits of knowledge if it is to remain practical.[95]

Gilbert Ryle's ghost in the machine argument, which is a rejection of Descartes' mind–body dualism, can provide a contemporary understanding of the soul/mind, and the problem concerning its connection to the brain/body.[96]

Psychologist James Hillman's archetypal psychology is an attempt to restore the concept of the soul, which Hillman viewed as the "self-sustaining and imagining substrate" upon which consciousness rests. Hillman described the soul as that "which makes meaning possible, [deepens] events into experiences, is communicated in love, and has a religious concern", as well as "a special relation with death".[97] Departing from the Cartesian dualism "between outer tangible reality and inner states of mind", Hillman takes the Neoplatonic stance[98] that there is a "third, middle position" in which soul resides.[99] Archetypal psychology acknowledges this third position by attuning to, and often accepting, the archetypes, dreams, myths, and even psychopathologies through which, in Hillman's view, soul expresses itself.

Neuroscience as an interdisciplinary field, and its branch of cognitive neuroscience particularly, operates under the ontological assumption of physicalism. In other words, it assumes that only the fundamental phenomena studied by physics exist. Thus, neuroscience seeks to understand mental phenomena within the framework according to which human thought and behavior are caused solely by physical processes taking place inside the brain, and it operates by the way of reductionism by seeking an explanation for the mind in terms of brain activity.[102][103]

To study the mind in terms of the brain several methods of functional neuroimaging are used to study the neuroanatomical correlates of various cognitive processes that constitute the mind. The evidence from brain imaging indicates that all processes of the mind have physical correlates in brain function.[104] However, such correlational studies cannot determine whether neural activity plays a causal role in the occurrence of these cognitive processes (correlation does not imply causation) and they cannot determine if the neural activity is either necessary or sufficient for such processes to occur. Identification of causation, and of necessary and sufficient conditions requires explicit experimental manipulation of that activity. If manipulation of brain activity changes consciousness, then a causal role for that brain activity can be inferred.[105][106] Two of the most common types of manipulation experiments are loss-of-function and gain-of-function experiments. In a loss-of-function (also called "necessity") experiment, a part of the nervous system is diminished or removed in an attempt to determine if it is necessary for a certain process to occur, and in a gain-of-function (also called "sufficiency") experiment, an aspect of the nervous system is increased relative to normal.[107] Manipulations of brain activity can be performed with direct electrical brain stimulation, magnetic brain stimulation using transcranial magnetic stimulation, psychopharmacological manipulation, optogenetic manipulation, and by studying the symptoms of brain damage (case studies) and lesions. In addition, neuroscientists are also investigating how the mind develops with the development of the brain.[108]

Physicist Sean M. Carroll has written that the idea of a soul is incompatible with quantum field theory (QFT). He writes that for a soul to exist: "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."[109]

Some theorists have invoked quantum indeterminism as an explanatory mechanism for possible soul/brain interaction, but neuroscientist Peter Clarke found errors with this viewpoint, noting there is no evidence that such processes play a role in brain function; Clarke concluded that a Cartesian soul has no basis from quantum physics.[110]

In 1901 Duncan MacDougall conducted an experiment in which he made weight measurements of patients as they died. He claimed that there was weight loss of varying amounts at the time of death; he concluded the soul weighed 21 grams, based on measurements of a single patient and discarding conflicting results.[113][114] The physicist Robert L. Park has written that MacDougall's experiments "are not regarded today as having any scientific merit" and the psychologist Bruce Hood wrote that "because the weight loss was not reliable or replicable, his findings were unscientific."[115][116]