= Requires a certain "balance" =
* The knife is capable of killing certain people (those of Jacob and MiB's ilk), but it must be wielded by somebody with the appropriate "scale" inside them.
The below bullets contradict the commonly accepted belief that Dogen merely sent Sayid after The Man in Black in order to get rid of him. This hasn't been explicitly confirmed, and thus these bullets are plausible.
* When Dogen gave Sayid the knife to kill the Man in Black, he told Sayid to "Prove it", in reference to Sayid's claim that he was not evil.
* Sayid stabbed The Man in Black, as instructed, but nothing happened. This is because Dogen was correct; Sayid is evil.
*The Dogen's knife was used incorrectly. Sayid didn't stab the Man in Black correctly and/or he allowed the Man in Black to speak before stabbing him, something expressly forbidden by Dogen.
** Dogen seemed to confirm Sayid and the Man in Black's version of events by the Temple pool, implying that the knife wasn't meant to work.
* The knife has some other purpose, tied to Dogen, that Sayid will discover later on, allowing him to escape the grasp of the Man in Black.
* The knife was merely something he happened to have laying around. If it was integral to killing the MIB, it would have been treated with more care than being left to one side, unguarded.
** Agreed. Dogen made no effort to pick it up or protect it when Sayid just dropped it casually on the ground. Dogen just had a knife hidden away for a time when a knife would come in handy.
** The knife was hidden and wrapped because it was an antique, or treasured relic, but in the confusion of MiB's threat upon the Temple, it was the only weapon-like object which Dogen KNEW nobody else would have grabbed.
*** The Knife was actually hidden away because Dogen would have been tempted to use it,(the fight with Sayid), which if did use and killed someone, his connection with the protective ash barrier around the temple would have been broken.
* It is an allusion to the Æsahættr; the knife in Philip Pullman's His Dark Materials which can cut through into alterate universes. The Æsahættr is the only weapon capable of killing God.
It is the Spear of Destiny. The Spear of Destiny or the Holy Lance refers to the lance that the Roman centurion used to pierce the side of Christ during the Crucifixion. Legend has it that whoever possesses it and understands its power, holds in his hand the destiny of the world for good or evil. The reference to Napoleon in Dr. Linus alludes to this as Napoleon attempted to obtain the lance after the battle of Austerlitz, but as it had been smuggled out of the city, he never got a hold of it. Also, Jacob refers to Richard as Ricardus, a Roman name.
==Knife used to stab Jacob==
The knife that Ben used to stab Jacob was not the same pugio as Dogen's Knife, but it was a similarly shaped dagger. It was provided by Flocke to Ben as they were approaching the Foot. It is uncertain where Flocke got it from, as there were several opportunities along the way where he could have recovered an old/special dagger. And it would not have looked out of place to be carrying it, as Locke was known for his knives. The point here is to question the special nature of killing with Dogen's special Knife, when another knife did the job on Jacob.
* also, Pugio users are told not top let MiB/Jacob speak, yet with Flocke and Ben in his home, Jacob speaks several times.
** It may be more to do with the desire within the assassin's soul. When Richard was sent to kill Jacob, he is only an accidental murderer, stricken with grief over his wife, seeking to save/rescue her. When Sayid is sent to kill MiB, he does it as a job, like he has many times in the past, cold and without emotion. But when Ben stabs Jacob, he does it with sincere anger and passion.
==Romulus and Remus mirror MiB and Jacob==
http//en.wikipedia.org/wiki/Romulus_and_Remus The founding myth of Rome tells us about two brothers Romulus and Remus. After different arguments concerning the foundation of Rome they get into a fight and finally, Romulus kills Remus. All in all, that suggests that neither Jacob is good nor MIB is evil. Like Cain/Abel and Jacob/Esau they are brothers and founding figures. Ultimately, one of them has to get out of the way, so that the other one can go his way.