Try a simple experiment. The results will be self-explanatory. Think of a sad event from your life. Similar feelings will soon follow, and with them memories of other such unpleasant episodes strung together through association. Scenes, odors, words, perhaps half-forgotten, will suddenly come upon you with new freshness.
Your thoughts will activate the appropriate feelings. Beneath your awareness, however, they will also trigger the cells" ever-present memory imprints of stimuli received when those events occurred. There is, to some extent now, a cellular memory playback - and on the part of the entire body, the recognition of its state at that time.
If you pursue such sorrowful thoughts persistently you are reactivating that body condition. Think of one of the most pleasant events that ever happened to you and the reverse will be true, but the process is the same. This time the associated memories are pleasant, and the body changes accordingly.
Remember, these mental associations are living things. They are formations of energy assembled into invisible structures, through processes quite as valid and complicated as the organization of any group of cells. Comparing them with cells, they are of briefer duration, generally speaking, though under certain conditions this does not apply. But your thoughts form structures as real as the cells. Their composition is different in that no solidity is involved in your terms.
As living cells have a structure, react to stimuli and organize according to their own classification, so do thoughts. Thoughts thrive on association. They magnetically attract others like themselves, and like some strange microscopic animals they repel their “enemies,” or other thoughts that are threatening to their own survival.
Using this analogy, your mental and emotional life forms a framework composed of such structures, and these act directly upon the cells of your physical body.