Descriptions of thoughtforms and the mental body come from Theosophists Annie Besant and C. W. Leadbeater who were both very influential in the shaping of modern psychic folklore:

"The mental body is an object of great beauty, the delicacy and rapid motion of its particles giving it an aspect of living iridescent light, and this beauty becomes an extraordinarily radiant and entrancing loveliness and the intellect becomes more highly evolved and is employed chiefly on pure and sublime topics. Every thought gives rise to a set of correlated vibrations in the matter of this body, accompanied with a marvelous display of color, like that in the spray of a waterfall as the sunlight strikes it, raised to the nth degree of color and vivid delicacy. The body under this impulse throws off a vibrating portion of itself, shaped by the nature of the vibration -- as figures are made by sand on a disk vibrating to a musical note -- and this gathers from the surrounding atmosphere matter like itself in fineness from the elemental essence of the mental world. We have then a thought-form pure and simple, and it is a living entity of intense activity animated by the one idea that generated it. If made of finer kinds of matter, it will be of great power and energy, and may be used as a most potent agent when directed by a strong and steady will....
Each definite thought produces a double effect-a radiating vibration and a floating form. The thought itself appears first to clairvoyant sight as a vibration in the mental body, and this may be either simple or complex....

If a man's thought or feeling is directly connected with someone else, the resultant thought-form moves toward that person and discharges itself upon his astral and mental bodies. If the man's thought is about himself, or is based upon a personal feeling, as the vast majority of thoughts are, it hovers round its creator and is always ready to react upon him whenever he is for a moment in a passive condition....

Each man travels through space enclosed within a case of his own building, surrounded by a mass of the forms created by his habitual thought. Through this medium he looks out upon the world, and naturally he sees everything tinged with its predominant colors, and all rates of vibration which reach him from without are more or less modified by its rate. Thus until the man learns complete control of thought and feeling, he sees nothing as it really is, since all his observations must be made through his medium, which distorts and colors everything like badly-made glass.

If the thought-form be neither definitely personal nor specially aimed at someone else, it simply floats detached in the atmosphere, all the time radiating vibrations similar to those originally sent forth by its creator. If it does not come into contact with any other mental body, this radiation gradually exhausts its store of energy, and in that case, the form falls to pieces; but if it succeeds in awakening sympathetic vibration in any mental body near at hand, an attraction is set up, and the thought-form is usually absorbed by that mental body.

To this picture of the mental body, Yogi Ramacharaka adds a further description of the mental world as such:

Places and localities are often permeated by the thought of persons who formerly lived there, who have moved away or died many years ago. The occultist knows that this thought-atmosphere of a village, town, city, or nation is the composite thought of those dwelling in it or whom have previously dwelt there. Strangers coming into the community feel the changed atmosphere about it, and, unless they find it in harmony with their own mental character, they feel uncomfortable and desire to leave the place. If one, not understanding the laws operating in the thought world, remains long in a place, he is most likely to be influenced by the prevailing thought-atmosphere, and in spite of himself a change begins to be manifest in him and he sinks or rises to the level of the prevailing thought....

In the same way dwellings, business-places, buildings, etc., take on the predominant thought of those inhabiting them or who have dwelt in them.

An example of the perception of thoughtforms is provided by the famous medium Eileen Garrett:

One sees lines and colors and symbols. These move, and one is wholly concentrated on them and their movement. I say "symbols" here for want of a better word. I frequently see curving lines of light and color that flow forward in strata, and in these strips or ribbons of movement there will appear other sharply angled lines that form and change and fade like arrow heads aimed and passing in various directions. And in this flow of energy that is full of form and color, these arrow heads will presently indicate the letter H. Each line of the H will be an independent curve, and their combination will not remain identifiable for very long. But I shall have caught it; and holding it suspended in awareness, I continue to watch the process develop and unfold. Soon a rapidly drifting A appears in the field of concentration, and then, let us suppose, an R; and presently I have gathered the word HARRY out of the void, either as a proper name or as a verb temporarily without either subject or object. Whether it is actually a noun or a verb will depend upon the context of the perception as a whole.

This process is infinitely rapid. But I have achieved an alertness of attention, of awareness, of being, which is equal to this rapid flow of immaterial line and color and symbol, and out of this alertness, poised above the flowing stream of differentiated energy, I gather a message with a meaning -- a message which has come to my consciousness out of the objective world as factually as the reflected light from the distant Moon may reach my consciousness by way of my sense of sight.

The existence of the mental world implies a view of nature incorporating meaning as well as mechanism. We are no longer dealing with blind forces bouncing aimlessly throughout the universe. The substance of the mental world is imbued with purpose. Minds, or monads, are constantly emitting radiation of an intelligent nature. Every thought may be thought of as an active spiritual force. Iconoclastic researcher Andrija Puharich, M.D., has coined the term inergy, meaning "intelligent energy" to refer to this realm of spirit or thought.