John Heinrich Zopfius in his Dissertation on Serbian Vampires (1733) says: "Vampires issue forth from their graves in the night, attack people sleeping quietly in their beds, suck out all the blood from their bodies and destroy them. They beset men, women and children alike, sparing neither age nor sex. Those who are under the fatal malignity of their influence complain of suffocation and a total deficiency of spirits, after which they soon expire. Some who, when at the point of death, have been asked if they can tell what is causing their decease, reply that such and such persons, lately dead, have risen from the tomb to torment and torture them."
Scoffern in his Stray Leaves of Science and Folk Lore writes: "The best definition I can give of a vampire is a living, mischievous and murderous dead body. A living dead body! The words are idle, contradictory, incomprehensible, but so are vampires."
Horst defines a vampire as "a dead body which continues to live in the grave, which it leaves, however, by night for the purpose of sucking the blood of the living, whereby it is nourished and preserved in good condition, instead of becoming decomposed like other dead bodies."
A demon has no body, although for purposes of his own he may energize, assume, or seem to assume one, but it is not his real and proper body. So the vampire is not strictly a demon, although his foul lust and horrid propensities be truly demonic and of hell. Neither may the vampire be called a ghost or phantom, strictly speaking, for an apparition is intangible. The vampire has a body and his craving for blood is to obtain sustenance for that body. He is neither dead nor alive; but living in death. He is an abnormality; the androgyne of the phantom world; a pariah among the fiends. How fearful a destiny is that of the vampire who has no rest in the grave but whose doom it is to come forth and prey upon the living.
In the first place it may briefly be inquired how the belief in vampirism originated. The origins, although of course very shadowy, may probably be said to go back to the earliest times when primitive man observed the mysterious relations between soul and body. The division of an individual into these two parts must have been suggested by his observation, however crude and rough, of the phenomenon of unconsciousness as exhibited in sleep, and more particularly in death. He cannot but have speculated concerning that something, the loss of which withdraws man forever from the living and waking world. He was bound to ask himself if there was any continuance, in any circumstances at present veiled from him, of that life and personality which had obviously passed elsewhere. The question was an eternal one. It was, moreover, a personal one which concerned him most intimately since it related to an experience he could not hope to escape.
The Vampire: His Kith & Kin (1928)