If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them.

Thoreau’s line appears in the concluding chapter of *Walden* (1854), a work shaped by his experiment in simple living at Walden Pond (1845–1847) and by his broader critique of conventional ambition and material “success.” In the “Conclusion,” he turns from describing daily life to urging readers toward deliberate, self-directed living. The image of “castles in the air” acknowledges the value of lofty ideals and imaginative aspiration—things often dismissed as impractical—while insisting they should be made real through disciplined action. The sentence functions as a final exhortation: keep the vision, but ground it in concrete choices and labor.

The quote reframes daydreams and idealism as not only permissible but necessary: “castles in the air” belong in the realm of imagination, where bold plans originate. Thoreau’s point is not to abandon such visions as futile, but to treat them as the proper starting point. The second sentence supplies the ethical imperative—translate aspiration into lived reality by “put[ting] the foundations under them.” In Thoreau’s philosophy, this means aligning one’s daily practices with one’s highest values, resisting social pressures that substitute busyness or wealth for meaning. The line captures his blend of romantic idealism and practical self-reliance.