Logic is a fundamental system of rules which the world adheres to. The majority of beings exist within Logic, but there are some exceptions. Logic is comprised of the four elements - water, earth, fire, air; and the Two Origins - light and dark.
As a general rule, all things, both inanimate and alive, exist within Logic. Arts were created in the ancient past to combat a great threat which existed outside of Logic. An example of how this works would be with a bullet. A bullet that exists within Logic would not harm a being outside of it, but a bullet created by sorcery could harm said being. The Black Beast existed outside of Logic, and because of that, it was able to easily shrug off nuclear blasts aimed directly at it. It was for this reason that Ars Magus and its variants were created during the Dark War, as there existed no other way of even scratching the Beast.
The lycanthrope were created by sorcery, meaning that they exist outside of Logic - it is for this reason that Valkenhayn R. Hellsing fought the Black Beast barehanded. Vampires also exist outside of Logic, such as Rachel Alucard.