What does the phrase, “there is properly no history, only biography” mean?

I found Ralph Waldo Emerson's famous words, “there is properly no history, only biography,” in his “Essay I” being quoted in the article titled “Keeping the dream alive: A biography,” appearing in June 21, 2012 Time magazine. The article deals with the collapse of American Dream, and “there is properly no history ...” comes in the following sentence:

“There is the crisis of our time. The American Dream may be slipping away. We have overcome such challenges before. To recover the Dream requires knowing where it came from, how it lasted so long and why it matters so much.

Emerson once remarked that there is properly no history, only biography. This is the biography of an idea, one that made America great. Whether that idea has much of a future is the question facing Americans now.”

The author, Jon Meacham seems to relate the word, “the biography of an idea” to American Dream, but I don’t get a clear idea about the phrase, “there is properly no history, only biography” meant by Emerson.

What does it mean in specific connection with “American Dream”?

By the way, what is the function of “properly” used as an adverb here? Does it mean “in proper (exact) sense, there is no (abstract string of) history, but for (individual) biography (of heroes or historic characters)?

Emerson was echoing the Great Man Theory of history espoused by his contemporary, Scottish historian Thomas Carlyle, who said:

“The history of the world is but the biography of great men.”

The American Dream is the national ethos of the United States. To quote Wikipedia:

The idea of the American Dream is rooted in the United States Declaration of Independence which proclaims that "all men are created equal" and that they are "endowed by their Creator with certain inalienable Rights" including "Life, Liberty and the pursuit of Happiness."

Meacham is arguing that the American Dream isn't simply an abstract ideal, but is a grand idea that has had a life of its own; and its biography is the history of the United States.

I would interpret this as saying that, as you suggest, "properly" is used to mean in the strict sense of the word. He is alluding to the fact that true history is a complete and unbiased account of the actual events that took place during some period in the past, but it is impossible for anyone to accomplish this because anyone attempting it will knowingly or unknowingly, by omission, inclusion, or interpretation, introduce their own biases and thus their account will be more a biographical account than an historical one.