The following dialogue contains ten arguments. Translate each into symbolic form and then use the eighteen rules of inference to derive the conclusion of each.

Is This the End?

Brian and Molly are at the memorial service of a mutual friend who had died suddenly the week before. “I’m still shocked to think that Karl is gone,” Molly says.

“I know you were quite close to him,” Brian says. “But do you think in some sense Karl could still be with us? I mean do you think there could be such a thing as postmortem persistence of consciousness—life after death, as most people say?”

“I wish there were,” Molly replies, “and that’s what makes death so tragic. As I see it, the mind is totally dependent on the brain, and if that’s so, when the brain dies, the mind dies. If the mind dies, then consciousness dies, too. Thus, if the brain dies, then consciousness dies—which means there’s no life after death.”

“But what makes you think that the mind is totally dependent on the brain?” Brian asks.
“Our day-to-day experience provides lots of evidence,” Molly replies. “If you drink alcohol, your mind is affected. If you smoke marijuana, your mind is affected. If your mind is affected by these things, then you have firsthand experience that the mind is dependent on the brain. Thus, if you either smoke marijuana or drink alcohol, then you have firsthand experience that the mind is dependent on the brain.”

“So the mind is affected by the brain. Anyone with ordinary sensation knows that,” Brian retorts. “If your eye receives a visual stimulus, then that stimulus is sent to the brain and your mind is affected. If your ear receives an auditory stimulus, then your mind is affected. Thus, if either your eye or your ear receives a stimulus, then your mind is affected. But that doesn’t prove that the mind is necessarily dependent on the brain. And there are lots of reasons for saying that it isn’t.”

“What reasons are those?” Molly asks.
“Well, we learned about Plato in Introduction to Philosophy,” Brian replies. “And Plato held that the mind can conceive ideal objects such as perfect justice and perfect triangularity. Now, if either of these concepts came through the senses, then perfect ideals exist in nature. But no perfect ideals exist in nature. And if the concept of triangularity did not come through the senses, then the mind produced it independently of the brain. But if that is the case or the concept of triangularity is innate, then the mind is not necessarily dependent on the brain. I’ll leave the conclusion up to you.”

“Very interesting,” Molly replies, “but I question whether the mind is really capable of conceiving ideal objects such as perfect justice and perfect triangularity. For me, these things are just words. But there are other reasons for thinking that the mind is necessarily dependent on the brain. Consider the visual cortex. The visual cortex is part of the brain. If the visual cortex isn’t stimulated, there is no visual sensation. But if visual sensation occurs only if the visual cortex is stimulated, and if the visual cortex is part of the brain, then visual sensation is dependent on the brain. And if that is true and visual sensation is a function of the mind, then the mind is necessarily dependent on the brain. Therefore, if visual sensation is a function of the mind, then the mind is necessarily dependent on the brain.”

“Furthermore,” Molly continues, “there are many cases where strokes have caused loss of memory, and also loss of speech. But if remembering is a mental function, then if the mind is not necessarily dependent on the brain, then strokes do not cause loss of memory. Therefore, if remembering is a mental function, then the mind is necessarily dependent on the brain.”

“It may indeed be the case,” Brian replies, “that memory—or at least certain kinds of memory—are dependent on the brain. And the same may be true of sensation. But that doesn’t prove that consciousness as such is brain dependent. It seems to me that consciousness as such is a nonmaterial process, and that it can occur only in a non material entity, such as a soul. And if those two claims are true and the soul is immortal, then consciousness survives the death of the body. Thus, if the soul is immortal, then consciousness survives the death of the body.”

“If memory goes with the brain,” Molly replies, “then I wonder if the consciousness you speak of is in any way your consciousness. But setting that aside, are there any reasons for thinking that the soul is immortal?”

“I think there are,” Brian replies. “If the soul is nonmaterial, then it has no parts, and if it has no parts, then it can’t come ‘a-part’—in other words it can’t disintegrate. And if it can’t disintegrate, then if nothing can destroy it, then it is immortal. But the soul can be destroyed only if God destroys it, and God does not destroy souls. Therefore, if the soul is nonmaterial, then it is immortal. I think Leibniz invented that argument.”

“Fine,” Molly says. “But what makes you think that you have a nonmaterial soul in the first place?”

“Well,” Brian replies, “according to Descartes, I am essentially either a mind or a body. But if I can doubt that I have a body, then I am not essentially a body. And I can doubt that I have a body. For example I can imagine that I am in The Matrix, and that all of my sensations are illusions. If I am essentially a mind, then if the essence of mind is to be nonextended, then I am a nonextended substance. But the essence of mind, being different from the essence of body, is to be nonextended. And if I am a nonextended substance, then I am (or have) a nonmaterial soul. Therefore, I am (or have) a nonmaterial soul.”

“Your argument is so abstruse that I don’t find it very persuasive,” says Molly, as she scratches her head. “I think the evidence is overwhelming that humans are the product of biological evolution, and if that is true and humans have souls, then there is a point in the course of evolution where humans either received or developed a soul. But there is no evidence that humans ever received a soul. Also, there is no evidence that humans ever developed a soul. Therefore, humans do not have souls.”

“Wow, that sounds pretty far out,” Brian replies. “Well, it looks like the service is ready to start, so we’ll have to hang this up. But maybe we can continue it at a later date.”
“Maybe we can,” Molly replies.