The Mastery of Destiny is a short book of ten essays in which James Allen sets out to dissolve the old quarrel between fate and free will. He argues that both are real, but that they are two aspects of a single principle: the law of cause and effect working in the moral world. A person freely chooses what causes to set going, yet cannot choose, alter, or avert the effects that follow. In this sense destiny is something a person has issued the mandate for, even without knowing it.
From this Allen draws his picture of character. Characters are not handed out ready-made at birth; they are built up, deed by deed, as an accumulation that hardens into settled habits of mind. Because character is a fixed combination of deeds, it carries its own results within it, so that what befalls a person is in large part the reflection of what that person has been and done. Life, on this view, is a school in which all are slowly learning the lessons of wisdom through their own sowing and reaping.
Much of the book is practical. Allen treats self-control as a science, mirroring the steps of the natural scientist (observation, experiment, classification, deduction, knowledge) with inward steps of introspection, self-analysis, adjustment, righteousness, and pure knowledge. He then turns to the training of the will, which he insists is not mysterious at all: it is developed by attacking one's own weak points through seven plain rules, from breaking bad habits to controlling the mind. Thoroughness, doing little things as if they were the greatest things in the world, is held up as the mark of a strong and useful life.
The later essays move from outer conduct to inner discipline. Mind-building is compared to building with bricks, where each thought is a brick and a sound life rests on four moral principles: justice, rectitude, sincerity, and kindness. Concentration is described as the bringing of a well-controlled mind to whatever has to be done, cultivated only through the daily task itself rather than through artificial methods. When concentration is joined to aspiration, it becomes meditation, which Allen carefully separates from idle reverie and ties to the purification of the heart.
The book closes on purpose and accomplishment. Allen presents purpose as highly concentrated thought, the keystone of achievement, and claims that all things at last yield to a fixed and lawful resolve. The final essay treats joy as the natural reward of work faithfully done and of self conquered. Its summary of the whole argument is plain: the price of life is effort, the height of effort is accomplishment, and the reward of accomplishment is joy.