Thoreau's color symbols are part of an elaborate system of symbolic imagery he used to express the stage of controllable insight at which the spiritual and moral in nature could be conveyed. But they are especially important because they show the degree of originality and technical refinement he could reach. Emphasizing the sensible properties of natural phenomena allowed him both to present nature directly, as his theory dictated, and artistically to strengthen an appeal designed to be above the level of nature-as-fact. But by making certain of these properties symbolic in themselves, he could increase the range and precision of his expression. Five colors emerged as major symbols because they fitted into his basic system so well. Green, the spring and summer color, he used to stand for organic life activity, for birth and growth. With white he symbolized purity and spirituality, carefully avoiding its associations with winter. Blue, the color of unclouded sky and water, he used to represent the esthetic atmosphere of meditation. With yellow, his sun color, he showed spiritual cause and material effect. And red, his most personally significant color symbol, he used intensively to stand for heroism, strength, and spiritual fruition.