Thought is a vital, living force, the most vital, subtle and irresistible force that exists in the universe. The thought-world is more real relatively than this physical universe. Thoughts are living things. Every change in thought is accompanied by vibration of its matter (mental). Thought as force needs a special kind of subtle matter in its working.
Mind assumes the form of anything it contemplates. When you think of an object, your mind shapes itself into the form of that object. When you change your thought, your mind also changes its shape. Many modifications continually arise in the mind. Your thoughts rapidly change. Your mind also changes its shape rapidly. Every moment, mind is continually creating hundreds of these thought-forms and continually dispersing them again. It never holds on steadily to one thought-form for some time.
Every thought has a certain name and a certain form. Form is the grosser and name the finer state of a single manifesting power called thought. But, these three are one. It is the unity in trinity, the three degrees of existence of the same thing. Wherever the one is, the others also are there. Suppose your mind is now perfectly calm, entirely without thought. Nevertheless, as soon as thought begins to rise, it will immediately take name and form. Thus you find that every idea that man has, or can have, must be connected with a certain word as its counterpart. Language is different, but thought is one. Mental image is the same in all.
Thought is subtle matter. A thought is as much solid as a piece of stone. You may die, but your thoughts can never die. They have form, size, shape, color, quality, substance, power and weight. The stronger the thoughts, the earlier the fructification. Thought is focused and given a particular direction and, in the degree that thought is thus focused and given direction, it is effective in the work it is sent out to accomplish.
Thought Gives Health
If you entertain healthy thoughts, you can keep good health. If you hold on to sickly thoughts in the mind, thoughts of diseased tissues, thoughts of weak-nerves, thoughts of improper functioning of organs or viscera, you can never expect good health, beauty and harmony. The body is the product of the mind. If you hold on vigorous thoughts in the mind, then the physical body also will be vigorous. Evil thoughts of all kinds befoul and infuriate the mind and, if persisted in, will become veritable diseases and maimings of the mind, incurable during the period of life. The strength of your body, the strength of your mind, your success in life and the pleasures you give to others by your company, all depend on the nature and quality of your thoughts. So think wise and be wise! :)
Mind – Its Mysteries and Control by Sri Swami Sivananda