Physical pain is perceived by
nociceptors. According to
Wikipedia, "a nociceptor is a sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain." Most nociceptors are located on human skin. There are about 20 nociceptors per square millimeter of skin. Overall, a human contains about 40 million nociceptors. Only 3 million of them are responsible for sensing noxious heat.
We can define one
pain unit (pu) as the experience created by one nociceptor sending maximal-strength signal to the brain. Similar pain may be experienced by tens of thousands of nociceptors sending lower-strength signals to the brain.
An example of a
mild pain is an injection for blood test or vaccination -- only 2 to 3 mechanical nociceptors are pierced by the needle. Thus the pain is about 2 to 3 pain units.
An example of
severe pain is amputation of a digit of a small finger. Similar pain would be experienced if someone's digit of a small finger is submerged in boiling water. That pain has a magnitude of 1,000 to 1,500 pain units corresponding to the number of nociceptors affected.
An example of
extremely severe pain is amputation of a foot. Similar pain would be experienced if someone's foot is submerged in boiling water. That pain has a magnitude of 60,000 to 100,000 pain units corresponding to the number of nociceptors affected. Prior to discovery of anesthetic in 1840s, such surgeries took on average half a minute.
Falling into hot acidic spring is an example of
unimaginable pain -- far beyond most other extremely severe pain. This pain has magnitude of 4,000,000 to 5,000,000 pain units corresponding to the total number of thermal and chemical nociceptors. The most extreme pain would last about 8 seconds after which the nociceptors stop functioning. Residual pain of tens of thousands of units lasts about 3 minutes.
Pain of 250,000,000 units is
infernal pain. No one can experience that pain during their lifetime.
Many people do not believe in Afterlife, yet it can not be disproved.