“What we have said in the introductory chapter and elsewhere, of mediums and the tendency of their mediumship, is not based upon conjecture, but upon actual experience and observation. There is scarcely one phase of mediumship, of either kind, that we have not seen exemplified during the past twenty-five years, in various countries.
India, Thibet, Borneo, Siam, Egypt, Asia Minor, America (North and South), and other parts of the world, have each displayed to us its peculiar phase of mediumistic phenomena and magical power.
Our varied experience has taught us two important truths, viz.: that for the exercise of the latter, personal purity and the exercise of a trained and indomitable will-power are indispensable; and that spiritualists can never assure themselves of the genuineness of mediumistic manifestations, unless they occur in the light and under such reasonable test conditions as would make an attempted fraud instantly noticed.
For fear of being misunderstood, we would remark that while, as a rule, physical phenomena are produced by the nature-spirits, of their own motion and to please their own fancy, still good disembodied human spirits, under exceptional circumstances, such as the aspiration of a pure heart or the occurrence of some favoring emergency, can manifest their presence by any of the phenomena except personal materialization.
But it must be a mighty attraction indeed to draw a pure, disembodied spirit from its radiant home into the foul atmosphere from which it escaped upon leaving its earthly body.”
H. P. Blavatsky
The Mark Of God: Understanding Your "I AM Presence" 