Here’s an interesting quote from William Walker Atkinson:

“That great writer on the power of the mind – Prentice Mulford, has summed up much of his philosophy in the statement: “Thoughts are Things”. In these words he gave expression to a mighty truth, which, if fully apprehended by mankind, would revolutionize the world.

Thought is not only a dynamic force, it is a real thing, just as is any other material object. Thought is merely a finer form of matter, or grosser form of mind, mind but a finer form of matter. There is but one substance in nature, but that substance has many forms, ranging from the most material (so-called) forms, to the highest form – Spirit.

When we think, we send out vibrations of a fine ethereal substance, which is as real as the finer vapors or gasses, the liquids, the solids. We do not see thought – neither do we see the finer vapors or gasses. We cannot smell or taste thought – neither do we smell or taste the pure air.

We can feel it however, as many can testify – which is more than we can say of the powerful magnetic vibrations of a mighty magnet, which, whilst exerting a force sufficient to attract toward it a piece of steel weighing a hundred pounds, is absolutely without effect upon us.

Its vibrations may pass through our bodies and exert is force on the steel, while we may be unaware of its existence. Light and heat send out vibrations of a lower intensity than those of thought, but the principle is the same. The evidence of the five senses is not absolutely necessary to establish the existence of a material substance of force.

The character of the thought vibrations sent out by us depends upon the nature of the thought itself. It thought had color (and some say that they have), we should see our fear and worry thought as murky, heavy, clouds hanging close to the earth; our bright, cheerful and happy, confident, “I can and I will” thought as light, fleecy, vapory clouds hanging close to the earth; our bright, cheerful and happy, confident, “I can and I Will” thoughts as light, fleecy, vapory clouds, traveling swiftly and mingling with others of their kind, forming fleecy cloud banks, high above the level of the dense, mephitic, foul exhalations produced by fear and worry “I can’t” thoughts.”

This description that Atkinson is doing of the character of the thoughts is very interesting and it’s not far off from the very mental states we may have of them.