A neurosurgeon who has performed more than 7,000 surgeries believes he has proof that people possess souls, pointing to everything from dying patients and conjoined twins to trees. 

When Michael Egnor, 69, began studying to become a neurosurgeon decades ago, he most certainly didn't believe in the concept.

'First of all, I didn't know what was meant by a soul. I would have thought back then that a soul was like a ghost, and I didn't believe in ghosts,' he told the Daily Mail ahead of the release of his book, The Immortal Mind.

'It's easier to study the brain like it was a computer. That is, bringing the soul into it makes it's more complex, it's less tangible,' he explained. 

In his mid-40s, Egnor started questioning the idea while working as a surgeon at Stony Brook University in New York, where he is still employed.

Doubt in his long-held beliefs began to creep in when he noticed patients who were missing significant portions of their brain appeared completely fine.

One pediatric patient grew up to be completely normal, despite, her brain consisting of 50 percent spinal fluid.

'Half of her head was just full of water,' Egnor recalled. 

When Michael Egnor, 69, began studying to become a neurosurgeon, he most certainly didn't believe in souls

'And I counseled her family that I didn't think she was going to do very well in life, that she obviously was going to have a lot of handicaps. And I was wrong.' 

The moment that really convinced him came when he went in to remove a tumor from the frontal lobe of a woman who was awake at the time. 

'She was perfectly normal through the whole conversation,' Egnor remembered. 'And here I was thinking: "Here I am, taking out a major part of the brain to cure this tumor, and she's perfectly all right when I'm doing it. So what is the relationship between the mind and the brain? How does that work?" 

'So I began to look rather deeply into the neuroscience of that question, and found that I wasn't the first person to ask it.' 

Egnor soon began to understand that the mind and brain weren't as interconnected as he once thought. 

'If you're missing half of your computer, it probably won't work very well, but that's not necessarily the case with the brain,' he told the Daily Mail.

'Our ability to reason, to have concepts, to make judgments, abstract thought - it doesn't seem to come from the brain in the same way.'

Egnor also found other case studies that he claims prove the existence of a soul, including the rare phenomenon of conjoined twins who share parts of a brain. 

Egnor found case studies that he claims prove the existence of a soul, including the rare phenomenon of conjoined twins who share parts of a brain, like Tatiana and Krista Hogan (pictured)

Another famous set of conjoined twins, Abby and Brittany Hansel, share a body but have their own heads and hearts. Abby is married to Josh Bowling, and they are all pictured together above

Abby and Brittany Hensel are pictured when they were younger. Regarding conjoined twins, Egnor said: 'They [have] different personalities, they have different senses of self'

In his book, he discussed the case of Canadian twins Tatiana and Krista Hogan, who share a vehicular bridge, which connects the two hemispheres of the brain. 

One twin controls one side of the body, while the other controls the opposite side. Despite sharing a brain, each has their own personalities, likes and dislikes, and more. 

Egnor said that conjoined twins are a 'composite of people who share abilities that normally would be characteristic of one person.' 

'That is, they share the ability to see through the other person's eyes, at least partially. And they share the ability to feel things... But in other ways, they're completely different. That is, they [have] different personalities, they have different senses of self,' he told the Daily Mail.

'Your soul is a spiritual soul, and her soul is a spiritual soul. So you have a spiritual part of yourself that you can't share with somebody else. That is, your spiritual self is yours alone, and that's the remarkable thing.'

Another famous set of conjoined twins, Abby and Brittany Hansel, share a body but have their own heads and hearts. Like Tatiana and Krista, they also have different personalities and even their own driver's licenses. 

'No conjoined twin situations are alike, but maintaining individuality as human beings does not appear to be the challenge we might have expected,' Egnor wrote in his book.

'That makes sense if the individual mind is a natural unity; it remains a unity even when sharing parts of a physical body with another mind.' 

Egnor is careful what he tells patients during surgery. 'You're really dealing with an eternal soul. You're dealing with somebody who will live forever, and you want the interaction to be a nice one'

'A tree has a soul, it's just a different kind of soul,' Egnor said. 'It's a soul that makes the tree alive. A dog has a soul. A bird has a soul'

When asked about identical twins - whose egg splits into two after fertilization - he said he didn't know the neurology behind it. Conjoined twins start as one egg that begins to split later in development but never fully separates. 

Egnor does believe that souls start at conception, an idea shared in part by the famous philosopher Aristotle. 

He believed the soul is the part of the body that makes it live. 

'So soul is what makes you talk and think, and what makes your heart beat, and your lungs breathe, and your body do physiology. All of that. Every characteristic of a living thing is its soul. So anything that is alive has a soul,' Egnor said. 

'So once I understood what was meant by a soul, which again, is just the thing that makes a body alive, that made sense to me. And so once I understood what Aristotle thought a soul was, I came to feel that that fit the science the best.'

However, a soul is not limited to humans, as animals and even trees have them, Egnore explained.

'A tree has a soul, it's just a different kind of soul,' he said. 'It's a soul that makes the tree alive. A dog has a soul. A bird has a soul.

'The difference with the human soul is that our soul has the capacity for abstract thought. It has the capacity to have concepts, to use reason, to make judgments, and to have free will.' 

Unlike the brain, the soul cannot be accessed through traditional tools, Egnor said. 'You can't cut it with a knife like you can cut the brain with a knife. And I believe that your soul is immortal' 

Egnor's book, The Immortal Mind, will be released on June 3

Unlike the brain, the soul cannot be accessed through traditional tools, he continued.

'You can't cut it with a knife like you can cut the brain with a knife. And I believe that your soul is immortal,' the father-of-four explained. 

With this in mind, Egnor is careful what he says to patients while they're under anesthesia or in comas.

'People in deep comas very often are still aware of what's going on around them. They're aware of conversations, and I've noticed that, too,' he said.

'If I'm in a room with a patient who is in a deep coma, I'm very careful about what I say, because it's rather common that if you say something that's frightening to the patient, their heart rate will rise. They're clearly having a reaction to it. 

'So, be careful what you say, you know. Never, never say something in a patient's presence, even if they're deep under anesthesia that you wouldn't want them to hear.

'You're really dealing with an eternal soul. You're dealing with somebody who will live forever, and you want the interaction to be a nice one.' 

In his book he references the case of Pam Reynolds, an American songwriter who had to have her head drained of blood and body chilled to operate on a bulge in her basilar artery. 

Reynolds came back from the operation perfectly fine, but she remembered staring at her body from above and talking to her ancestors, who told her it wasn't her time to die. 

Eventually, her ancestors forced to her to go back and she recalled how painful it was for her soul to reenter her body. 

'It was like diving into a pool of ice water... it hurt,' she said, according to a passage in Egnor's book.

Just because patients might be able hear him during surgery, he doesn't talk to them - even to convince their souls to stay during critical moments. 

'I don't know that I control whether their soul can come back or not,' he told the Daily Mail. 'I certainly pray to God that that he takes care of their soul, and that he comforts them and their family, and I always pray for the best for them.' 

Egnor's book, The Immortal Mind, will be released on June 3.