Introduction
            The belief fin supernatural beings is innate in man. The Thai people as a race call such supernatural beings by the generic word "phi", which includes both gods and devils. The phi, like man in a general sense, are of two classes, the good Phi and the bad phi. When the Thai came in contact with the highly hinduized Khmer or Cambodians in Central Thailand in the 12th century A.D. and had become a ruling race in that region, they adopted most of the Khmer hinduized cultures, especially the ruling class. Throughout subsequent centuries the Thai and The Khmer mixed racially and culturally to an appreciable degree. By this time the Thai were gradually becoming known as the Siamese and the old Thai word "phi" like its owners had also undergone a change in meaning. In the famous stone inscription of the great Siamese King Ramkamhang dated 1283 A.D. reference was made to the King of Khmer of that time as "phi fa" which literally meant the heavenly phi. Actually "phi fa" meant a divine kings which cult had been adopted by Siamese kings of the later periods. Instead of referring to a divine king as phi fa as hitherto, it has now changed into a "thep" or "thevade" from the Sanskrit "deva" and "devata" which mean a god or, literally, a shining one. It followed Thai all the good phi of the Thai had by now become thevada or gods in their popular use of the language. The generic word "phi" therefore, degenerated into a restricted meaning of bad phi. It now means a ghost, a devil or an evil spirit. Nevertheless the old meaning of phi in certain cases is not yet dead and still lingers in some expressions in the language. For instance, of any evil deed done in secret, we sometimes say as warning, "men never see the evil deed done but the phi does." In order not to divulge the source of any formula, especially a medicinal prescription which is effective the owner will say that the formula is "phi Bok", or told by a phi, so as to give it a sacred and mystical effect. The phi here is a good phi or a thevada
            The dividing line between gods and devils like men, is a thin one, which is a matter of varying degrees. Some gods are bad and some devils are good. There are, in fact, almost as many kinds of good and bad phi as there are of men. It follows therefore, that out of this phi there emerges a class whose position is on a border lime between the gods and the devils. They are called "Cho phi" which means a lord or prince phi but is sometimes also called thevada. Such supernatural beings, half phi and half thevada, form the subject of my paper.