Simple mortality is the concept that a person dies once and never exist again. Superimmortality is the concept that consciousness and ixperiencitness is produced by the structure and functioning of the body and not just one body. Many different bodies can produce the same or similar structures and functionings of matter so they can produce the same or similar consciousnesses with the same ixperiencitness. Simple mortality is based on the experiences that most people have in life where other people die and never come back to life. The universe is more complex than this, and our knowledge has increased so that we know that the brain alone can produce consciousness but it is not alone in producing consciousness. Many different brains can produce consciousness but it has been assumed, by many, that only one brain can produce one ixperiencitness. This belief has persisted because we can not see the consciousness and ixperiencitness in other brains.
Complex mortality is the consequence of considering rational experiments about what determines the same person over time. The problem with being the same person over time is that the same person does not have to have the same ixperiencitness unless so defined. How long can a person be dead and still be the same person. How much matter can be replaced and how fast and still be the same person? If a person is split into two even and equal pieces with equal amount of brain and then the lacking half of each one reconstructed with new matter but identical structure and functioning so they both produce the same identical consciousness, who will be the original person?