The Nature of Jacob and Nemesis

Following is conjecture on the nature of Jacob and Nemesis, from the Finale evidence.

PHYSICAL CHARACTERISTICS

• There is no evidence that Jacob or Nemesis have god-like or superhuman powers. So far the only characteristic they possess unexplained by the island or science seems to be their long, or immortal life. We are also unsure of the effect Jacob's touch has on humans while they are on the mainland.
  
• Jacob is a corporeal entity. He can be seen by humans because he inhabits a real living body.
• Conversely, Nemesis is a spirit. The evidence is that no one has yet seen him. Jacob doesn't count.
  
• At least a couple theories explain his ethereal quality. Perhaps he was killed at some point. Perhaps he is somehow the alter ego of Jacob personified. The reason he is a spirit may not be important.
  
• Nemesis resorts to physical possession of dead bodies, and controlling smoke to act or be seen in the real world. He can not possess live people. Notice that he must possess the body; he can not shape shift, or mimic a body, or he would not need a dead one.
  
• Jacob can recognize Nemesis either through a spiritual sense, or because he knows him so well after all these centuries.
• Their different physical make-ups go a long way to explain why they can not kill each other, although there is little evidence that Jacob wants to kill Nemesis in any case. If Nemesis is Jacob's personified alter ego, it would simply explain why he must convince a proxy to kill him; Nemesis does not wish to kill himself, especially if he does not inhabit another body at the time.