Art
If "nature" refers to the essences of the world unchanged by humans (e.g., the river, the air), then "art" refers to the application of human will to nature (e.g., a house, a statue, a picture). Note that Emerson wrote in a time before the human impact on the environment was fully understood.
Beauty
In Nature, Emerson reminds us that the ancient Greeks (e.g., Pythagoras) described the "world" (kosmos) as the embodiment of beauty (and order). Beauty arises from the primary forms of the world (e.g., the sky, the mountain, the tree, the animal), and their features (e.g., outline, color, motion, grouping) are inherently pleasurable to the human eye. In other words, beauty arises from our connection with nature.
Calvinism
As maintained and defended by the Puritan settlers of New England, Calvinism, which dominated New England religious culture until the late 18th century, emphasized the inherent depravity of human nature, and the election by God of only select sinners for salvation. In this Christian theology, neither the actions nor character of the individual affected his/her eternal fate, but only the gift of grace that could be conferred upon by God. However, New England Calvinism was also an evolving theology, and the Puritans introduced modified versions of the doctrine, such as the preparation of the soul for salvation.
Commodity
As discussed in Nature, Emerson understands commodities as one of the primary benefits of nature, in this case the raw materials and energies provided by nature for what we build, grow, and/or eat (i.e., the practical usefulness of nature). However, this only encompasses the most obvious and tangible of benefits of nature, which also include beauty, language, and discipline.
Correspondence
Correspondence refers to the connection between the mind (the inner world) and external nature (the outer world). In "The Poet," Emerson says, "The universe is the externalization of the soul." This omnipresent connection implied a monastically united universe where the physical and mental, the individual soul and oversoul/Reason/God, were two sides of the same coin.
Discipline
In Nature, Emerson understands nature as a discipline (among other things). Every property of matter (e.g., solidity, inertia, extension, figure) serves to educate us about 1) intellectual truths (e.g., about difference, likeness, order, the exercise of the Will/power) and 2) the nature of Reason (i.e., how the infinite variety of external forms of nature reflect and reference spiritual nature).
Experience
As described in "Experience," Emerson understood the term in its broadest sense as the sensations and states of mind in life unified by consciousness, which include both the mystical/spiritual and the sensuous/physical. In this way, experience is a bridge between the objective and the subjective, between nature and the soul. In the interactions between nature and the soul, the true meaning of life and reality are discovered.
Genius
Genius is the achievement of the self-reliant individual who believes in and lives by his/her own thought. However, such individuals demonstrate not just their own "wealth," but also the "common wealth" - that is, the genius is representative of the whole.
God
For Emerson, God is not the typical anthropomorphic God of historical and modern Christianity, but rather a universal soul (what he calls, "Reason" in Nature, and the "Over-Soul" in "Over-Soul") to which all of humanity and nature are connected, and from which the nature of Justice, Truth, Love, and Freedom emerge.
Historical Christianity
By "historical Christianity," Emerson meant organized and institutionalized Christianity, which he rejected most explicitly in the "Divinity School Address" as based on second-hand revelation and gospel. (Emerson never referred to the Bible as an authoritative source in his writing.) He instead favored a new theology based on the "moral sentiment" found in all human beings and religions, in which "God" could be found through actual, present, personal experience.
History - America
At the time when Emerson wrote, the history of America was considered superficial (i.e., without a substantial body of memorable events and cultural resources) compared with the history of Europe. (You may note, however, such a conception of "America" absented the history and culture of Native Americans, and specifically focused on the history of America after it was colonized by Europeans.) In the view of many of his countrymen, from the American Revolution to the 1830s, the history of America was the history of the westward movement of European people in the new world, of the application of European institutions, customs, and values to shape a new context and nature. Emerson rejected this perspective: he proposed that American history was the history of nature speaking through men (i.e., of their land as an expression of the soul in themselves based on correspondence), the discovery of a specifically American culture rooted in the nature of the new world, rather than of the history of European man domesticating nature in the new world according to old world (European) values and manners. His perspective influenced some of the major works of American literature, including Moby Dick, Leaves of Grass, and Walden, and helped to spur the development of an American school of philosophy by William James and John Dewy.
History - Individual
Emerson understood history not as a rule of cause-and-effect that determines the present (e.g., inheritance, family background), but rather as a set of circumstances and possibilities to be mastered by the individual in crafting his/her own destiny. People exist due to history (more specifically, their parents), but who/what they are is an open question.
Idealism
Idealism is a philosophy that proposes that reality is fundamentally a construction of the mind and thus immaterial. What is commonly referred to as the "real" is, to an Idealist, the "ideal" - the Idea or Soul whose appearance is merely a visible, imperfect expression. The ideal relates and unites the elements in the natural world to each other and humanity. While Emerson's idealism did not go so far as to deny the existence or reality of matter, he did emphasize the importance of the mind rather than matter, which led some critics to call him a "dualist" because of the limits of his idealism.
Language
In Nature, Emerson understands language as another use of Nature - in this case, as a vehicle of thought grounded in the natural world. He argues that 1) words are signs of natural facts (i.e., borrows from some material appearance, as evidenced in etymology - for example, "spirit" means wind and "supercilious" means the raising of the eyebrow), 2) natural facts are symbols of spiritual facts (i.e., every appearance in nature corresponds to a state of the mind, as with how we describe an enraged person as a lion, a cunning person as a fox, a lamb as innocence, lightness and darkness as knowledge and ignorance) and therefore concludes that 3) nature is a symbol of the spirit (i.e., the world is not simply significant because of the symbolic meaning we confer upon them, but are emblematic - nature is a metaphor for the human mind, as in proverbs like "a rolling stone gathers no moss" and "a bird in hand is worth two in the bush").
Nature
According to Emerson, nature (as opposed to grace, the state, the past/history, economics, race, sex, genetics, etc.) is the determining force and fundamental context of our lives. His definition of nature thus broadly includes: 1) a theory of the nature of things (i.e., the way things are, the underlying laws of the universe and the human mind), 2) a guide to life (including ethics, philosophy, art, language, and education), 3) an immediate physical experience of the world/God, and 4) everything that is not me/my soul/my consciousness (i.e., the external world, including flora and fauna in the environment, art, all other people, my own body).
Over-Soul
See entry for "God."
Reason
See entry for "God." Also, in Nature, Emerson distinguishes the understanding of intellectual truths (Understanding) from moral truths (Reason).
Religion
Emerson understood religion as the concrete, personal, and immediate feelings or experiences of the "moral sentiment" found in all human beings that provided insight into the perfection of the laws of the soul.
Science
Unlike the modern understanding of this term (i.e., as primarily referring to the natural and physical sciences), Emerson uses "science" to refer to both the natural/physical sciences and humanistic inquiry.
Self-Reliance
Elaborated upon in "Self-Reliance," the term refers to the chief virtue of the cultivated human who recognizes and lives by his/her inner genius (i.e., trusts his/her own thoughts) without being compromised by the influences and values of society.
Stoicism
Classical stoicism, as formulated by Zeno at the end of the fourth century BC and later elaborated upon by figures like Marcus Aurelius and Montaigne, was central to Emerson's thought. According to stoicism, nature - as opposed to tradition, authority, or the state - is the primary source of principles for an ethical life. To live in nature is to live in the present and to seize the day.
Transcendentalism
Initially an outgrowth of Unitarianism, Transcendentalism - as articulated and developed by Emerson (e.g., in Nature) and his followers, like Henry David Thoreau and Margaret Fuller - emerged as 1) an ethical and religious reformist movement that rejected "historical Christianity" in favor of a more direct connection with a universal soul (i.e., God or Reason), an impersonal force that operated according to "the moral law," grounded in everyday experiences with nature in the present; 2) an aesthetic, literary, and philosophical treatise molded by ancient and modern influences, including Idealism, Stoicism, German and English Romanticism (e.g., Goethe, Wordsworth, Carlyle), Skepticism (e.g., Hume), Biblical criticism (e.g., Herder, Schleiermacher), Eastern religion and philosophy, and the mystical philosophy of Emanuel Swedenborg; and 3) a social and political commentary that resisted established conventions (e.g., American slavery), sought modes of rethinking the relationship between humanity and the world, and engaged contemporary readers in the process of identity formation.
Understanding
In Nature, Emerson distinguishes the understanding of intellectual truths (Understanding) from moral truths (Reason).
Unitarianism
The American Unitarian Association was formed in 1825, as the culmination of a religious divide in New England between the then-dominant Calvinism, and the growing number of ministers in and around Boston who began to reject Calvinist doctrine for a more liberal and positive view of human nature by the mid-eighteenth century. Unitarianism differed from Calvinism in several ways, including: 1) Calvinism emphasized the inherent depravity of humanity and election by grace as central to salvation, whereas Unitarianism emphasized individual piety and ethical behavior as central to salvation; and 2) Calvinism worshiped "God in three persons" with a somewhat anthropomorphic God and divine Jesus, whereas Unitarianism worshiped a single God and contended that Jesus was not divine but rather the perfect "Son of Man."
