This study traces the developments of Thoreau's thoughts, in a vision of the universe and of man rooted in the transcendentalist current of thought in 19C New England, in order to scan what eclectical elements, especially the eastern and the western thoughts are involved in them. In the formative years of his philosophy, he was deeply influenced by the thoughts of Waldo Emerson's idealism, which was formulated on the basis of Plato's idealism, eclectically with Hindu philosophy, adapting Kantean terminology, the 'priori'. Emerson expounded the epistemology, 'Reason over Understanding ' as the only cognitive power intuitively to find the ultimate truth. He also formulated various concepts, such as 'the nature as the symbols of thoughts', 'over-soul', 'correspondence theories in universe/nature/man', 'ethical law identical with physical law', 'the theory of compensation', 'microcosm as individual', and so on. Generally until about mid-1840s, Thoreau accepted Emerson's idealism; however, he inclined more toword Hindu philosophy, especially yoga practice and its 'true knowledge'. He was deeply influenced by its asceticism and simple living, which formed his doctrine of smplicity. Since the later parts of 1840s, his epistemology gradually turned from the world of reason, the world of ideas, to the world of understanding, the world of the empirical. Ks search for truth became more scientific; and finally found the nature as ultimate reality, with its fundamentality, eternity, purity, and simplicity. On this practical ground he faced all the 19th Century issues, the cultures, the industrial, as well as the human rights. In this later process his methods was closer to Aristotelian as a western philosophy than to Plato; and influenced by Confucianism, especially in his theme of 'Self-Culture'.