As a Man Thinketh is a short argument about the moral force of thought. Allen's claim is simple but demanding: a person is shaped from within before that shape appears in action, habit, circumstance, achievement, or peace of mind.
The book begins with character. Allen treats actions as the visible fruit of hidden thought. What a person repeatedly allows into the mind becomes part of the person. In this view, noble character is not a gift of luck, and weak character is not fixed fate. Both grow through what is chosen, entertained, and practiced inwardly.
Allen then turns to circumstance. He does not simply say that every visible condition explains a person's whole character. In fact, he notes that outside conditions are too complicated for another person to judge so easily. His stronger point is that people often fight effects while preserving causes within themselves. Lasting change requires attention to the thoughts, desires, and habits that keep producing the same pattern.
The most practical image in the book is the mind as a garden. If it is cultivated, it can bring forth useful growth. If neglected, it still brings forth something, but not necessarily what the person would choose. The reader is asked to become a gardener of thought: removing what is useless or impure and cultivating what is clear, strong, and purposeful.
Purpose is the book's answer to drift. Allen argues that thought joined to a definite aim becomes forceful. Doubt and fear weaken effort, while directed thought strengthens achievement. The same pattern applies to ideals: people move toward the visions they sincerely hold and serve.
The final movement of the book is toward serenity. Calmness is presented as trained strength. A calm person has learned to govern thought, understand cause and effect in the inner life, and meet the world with steadiness. The book's essence is not that thought is magic. It is that thought is formative, and that the discipline of thought is the discipline of life.