The existence of entities has been studied and documented throughout history in many cultures and these can cause significant symptoms in those affected.
The best argument in favor of entity existence can be found in the fact that almost all traditional societies believed in the existence of entities and had developed methods for dealing with them, beginning in ancient times and continuing forth until today. Almost all shamanistic teachings have methods for the clearing of spirits and in many cultures this places spirit entity clearance at the same level of importance as surgery or paediatrics!
In Chinese medicine in the practice of acupuncture, among the 361 acupuncture points, we see the word KUEI (meaning discarnate spirit) making up part of the main or secondary name of 17 points, thus supporting the Chinese belief in the importance of the spirit in maintaining the health and well being of the body generally.
In many culturally specific Bibles, there can be found many unambiguous references to the clearance of spirits and entities and many religions have technical rituals dating from ancient times for taking care of the energetic "pollution" arising from someone's passing and measures taken to protect the living from that circumstance.
With this in mind, we can be sure that spirit entities are not simply some kind of odd theory made up to frighten people and that our modern Western Culture is at odds with history and all other cultures as it disregards the existence of spirits and entities to the detriment of those people who suffer mentally, emotionally and physically due to attachment meaning that many such people never receive the assistance that they so desperately need and thus never find relief.
The basic definition of an entity of spirit is a non-physical energy parasite with some consciousness of its own which attaches to your subtle energy body just as physical parasites (intestinal worms etc) do. The level of consciousness held by these beings varies with the individual as does their effect on the human (or animal) host and so the signs of spirit entity attachment are many and varied, ranging from physical aches and pains to paranoid delusion and complete spirit possession.
The origins of spirits and ghosts vary depending upon our belief system. For example, if we look at the beliefs of both the ancient Chinese and the ancient Indians, the consciousness is shattered at the time of death and this facilitates the ascension of some of our energetic bodies to the Spiritual realms, whilst others reincarnate into new physical vehicles. In Australian Aboriginals the Spirit is swept from the body and returns to its dreaming until it is time for it to be born again.
Sometimes, particularly in cases whereby death has been violent, the spirit is traumatized and disoriented and can miss it's "window" opening in which it can be drawn to "the light" and as such becomes trapped in the astral layer, leaving it unable to return to the Source without assistance, though they are often unaware of their trapped state and continue searching for the light which will take them to the other realms and people with a clear, bright light, (often those involved in Spiritual work or those with high abilities) commonly encounter spirit entities who have mistakenly been drawn to their light and at times become trapped in the magnetic aura of the live host before they are aware of their mistake.
Others commonly preyed upon are those suffering with addictions of any kind or obsessive compulsive behaviors. These people will be sought out by spirits and entities that had similar weaknesses in their physical lives so that they may continue their addiction "by proxy". Entities are similar to physical people in that they will tend to seek out others of like mind or interest to spend their time with and this can lead to difficulties for the unsuspecting individual. The personality does not die with the physical body and as such neither does the addiction until the soul has been able to move forward into healing.
Spirit beings can attach to both human beings and animals and at times they are not fussy as to which of these they attach to! Cases have been documented of both human hosts housing animal entities and vice versa and such beings may also attach to a home or possession to which they were particularly attached in life which can cause issues when the being still tries to exert their control here.
The Chinese call these entities KUEI, whilst Sanskrit calls them BHUTA (a word that translates to entity). Other kinds of Soul Fragmentation can occur when a person undergoes significant trauma (such as abuse or extreme emotional pressure) which can cause the soul of that individual to actually split or fracture leading to behaviour similar to those seen in people with Multiple Personality Disorder.
Human and animal Soul Fragments occupy a large proportion of the spirit entity attachments that clearers come across (up to 70%), however, there are all sorts of other non-physical beings that can and do, for whatever reason, attach to the human energetic body. Some of these are referred to as elementals, undines, devas, fairies, demons, aliens etc. Many of these beings are very tiny and as a result to not make much impact on the energies of their hosts, however, others have greater magnitude and can affect their host spectacularly.
