The single business of Henry Thoreau, during forty odd years of eager activity, was to dis

cover an economy calculated to provide a satisfying life. His one concern, which gave to his ramblings in Concord fields a value of high adventure, was to explore the true meaning of wealth. As he understood the problems of economics, there were three possible solutions open to him, to exploit himself, to exploit his fellows, or to reduce the problem to its lowest denominator. The first was quite impossible——to imprison oneself in a treadmill when the morning called to great adventure. To exploit one's fellows seemed to Thoreau's sensitive social conscience an even greater infidelity. Freedom with abstinence seemed to him better than serfdom with material well-being, and he was content to move to Walden Pond and set about the high business of living, "to front only the essential facts of life and to see what it had to teach." He did not advocate that other men should build cabins and live isolated. He had no wish to dogmatize concernig the best mode of living——each must settle that for himself. But that a satisfying life should be lived, he was virtually concerned. The story of his emancipation from the lower economics is the one romance of his life, and Walden is his great book. It is a book in praise of life rather than of Nature, a record of calculating economies that studied saving in order to spend more largely. But it is a book of social criticism as well, in spite of its explicit denial of such a purpose. In considering the true nature of economy he concluded, with Ruskin, that the cost of a thing is the amount of life which is required in exchange for it, immediatey or in the long run. In Walden Thoreau elaborated the text: "The only wealth is life."

The main idea of this paragraph is best expressed as ______.

A．problems of economics

B．Thoreau's philosophy

C．Walden, Thoreau's greatest book

D．how Thoreau saved money