The student will readily perceive that a thought-form can affect another person only if in the aura of that person there are materials capable of responding sympathetically to the vibration of the thought-form. In cases where the vibrations of the thought-form are outside the limits within which the person’s aura is capable of vibrating, the thought-form rebounds from it, and that with a force proportional to the energy with which it impinged upon it.
Hence the saying that a pure mind and heart are the best protection against inimical assaults, for a pure mind and heart will construct mental and astral bodies of fine and subtle materials, and these bodies cannot respond to vibrations that demand coarse and dense matter. If an evil thought, projected with malefic intent, strikes such a purified body, it will rebound and fly back along the magnetic line of least resistance, returning to and striking its projector. He, having matter in his mental and astral bodies similar to that of the thought-form he generated, is thrown into respondent vibrations, and suffers the destructive effects he had intended to cause another.
Thus “ curses “ [and blessings] come home to roost. From this arise also the very serious effects of hating or suspecting a good and highly advanced man; the thoughtforms sent against him cannot injure him, and they rebound against their projectors, shattering them mentally, morally, or physically. When as man thinks of himself as in some distant place, or wishes earnestly to be there, the thought-form, which he makes in his own image, appears in that place. Not infrequently such a form has been seen by others, and has sometimes been mistaken for the astral body or apparition of the man himself.
To make this possible, either the seer must have sufficient clairvoyance for the time to be able to see the thought-form, or the thought-form must have sufficient strength to materialise itself i.e.,, to draw round itself temporarily a certain amount of physical matter. The thought which generates such a form must necessarily be a strong one, and it therefore employs a large proportion of the matter of the mental body, so that though the form is small and compressed when it leaves the thinker, it usually expands to life-size before it appears at its destination.
Furthermore, a thought-form such as this, which must essentially be composed of mental matter, in very many cases will also draw round itself a considerable amount of astral matter. In taking on the astral form the mental elemental loses much of its brilliance, though its glowing colour may still be plainly visible inside the shell of lower matter which it assumes.
Just as the original thought ensouls the elemental essence of the mental plane, so the same thought, plus its form as a mental elemental, acts as the soul of the astral elemental. None of the consciousness of the thinker would be included in a thought-form such as that just described. When once sent out from him it would normally be quite a separate entity -not, indeed entirely unconnected with its creator, but practically so as far as the possibility of receiving any impression through it is concerned.
***From Arthur E. Powell – The Mental Body