The very nature of doubt is to "question success" (Merriam-Webster Handbook).
Doubt refers to a characteristic of thinking that makes us feel insecure, uncertain. The effort spent on doubt takes energy and is completely useless, because it does not motivate active action. The metaphor in this case can be a balloon pierced with a needle, from which air slowly escapes, but it is impossible to notice it visually. When we begin to doubt, we start a process of distorted perception, similar to a deflating balloon.
