Everything in the world of form is preceded by a thought. Thoughts are not in the world of form, form is not in the world of thought. But one actually precedes the other in all cases. If one wishes to affect form, one must work with cause, not effect. Thought is cause, form is effect. And when we work in the realm of thought, choosing a negative one or a positive one will influence the outcome.
Simply put, we live in a world of cause and effect i.e. for every cause there is an effect and every effect has to have a cause. Everything that happens in this world (form) is an effect, the cause being thoughts. These thoughts affect the whole universe. And each thought once generated and sent out becomes independent of the brain and mind and will live upon its own energy depending upon its intensity.