Transcendentalism is a philosophical movement that arose in the United States in the late 1820s. Like Romanticism, from which it sprang, it rejected tradition in favor of intuition, privileged nature over civilization, and trusted in the inherent goodness of the individual. It preached that all people are born possessing a divine spark. Prominent transcendentalists, most notably Emerson, advised that people should reject vitiated European book learning and turn inward to be led by God towards their divine purpose.
The transcendentalists have had the long lasting effect of putting a belief in individualism at the center of the American psyche. We are a country that favors the creative chaos of individualistic pursuit over subordinating oneself to tradition and authority.
Trusting in the guidance of the individual conscience has led to an enshrining of the notion of civil disobedience. Thoreau argued for the primacy of conscience in the fraught period before the Civil War when he wrote "Civil Disobedience," an essay that strongly supported people of conscience in disobeying unjust laws. This essay had a strong influence on Martin Luther King Jr.'s nonviolent protests against racism in the 1960s.
On the political front, the transcendentalists were at the forefront of advocating for abolition of slavery and women's rights in this country.
Writers like Emerson and, very prominently, Thoreau, also advocated turning to the simplicity and divinity of nature. Thoreau's Walden helped inspire the simplicity movement that has long been a part of the American character, pitting American's rugged and plain democracy against effete European ostentation. Further, the writing of transcendentalists like Emerson and Thoreau helped fuel the American preservation program, in which areas of wilderness have been set aside as valued national treasure.