What is the significance of the knife in occult practices?

[Brian]

Ive seen many pictures of altars that featured a skull, candle, and knife and some herbs or feathers. and on Wicca altars that had a knife in it as well among other items of similar fashion. im in the process of studying iconography and symbolism in occult, Wicca, necromancy and pagan rituals and id appreciate some assistance, as ive hit a snag and cannot find any good sources to point me on the right path. thanks again in advance