I agree with Khorianas on this one, as well as what many others have put down. You cast spells by retrieving energy, you can take this energy from different places e.g. Heat from your surrounding area to make a fireball, but you only need a little bit of heat as the energy you will then add to it makes it burn brighter and hotter. Makes an interesting case on nature magic though and it makes sense as to why only shamans or druids can use it, they are manipulating the spirits of or nature itself, they can do this as they have such a close connection with nature or the spirits of earth, fire etc. If others were to attempt this then the spells wouldn't work.
This also helps makes the point that you don't need hands or arms even to be a mage or warlock, You could direct the spells with your mind, although this would take a lot of practice and I daresay that it would cost extra energy if you wanted the magic to move in a certain direction without arms. Thats my opinion anyway no doubt some people will see it as wrong but anyway n.n