In Walden, Thoreau defines the necessities of life as follows:

By the words, necessary of life, I mean whatever, of all that man obtains by his own exertions, has been from the first, or from long use has become, so important to human life that few, if any, whether from savageness, or poverty, or philosophy, ever attempt to do without it.

For animals, he points out, food is often the only necessity of life, though they sometimes also seek shelter. For human beings faced with the New England climate, however, Thoreau adds two more necessities: clothing and fuel. It is only when people have food, shelter, clothing, and fuel that they are prepared to tackle philosophical problems with their full attention and observe what is around them without constantly worrying about their own survival.

Thoreau's determination to live a simple life, paring down his needs to the bare minimum, leads him to question every one of these necessities except food. He points out that shelter is often unnecessary, particularly in warm climates in the summer, and the same may be said of fuel, since there are many types of food which do not require cooking. As far as clothing is concerned, Thoreau quotes Darwin, who observed that the native people on Tierra del Fuego seemed to be warmer without it than Europeans were when fully dressed. He wonders whether it might be possible "to combine the hardiness of these savages with the intellectualness of the civilized man." However, Thoreau finally admits that clothes are a necessity, fortunately for him and for American literature, since it seems unlikely that his sojourn at Walden Pond would have lasted long if he had attempted to conduct his lifestyle experiment naked.