Naturally, we can see events in proper perspective; we know a period better than the active participants in it because we see its results, and because events disclose their real significance by what they produce, and the products can be seen only by those who come afterward and look back ... even of movements, motive, and incident, we often have a firmer grasp than did the men [and women] that were part of what we study.McLaughlin offers a primer on the sources preferred by historians.
Andrew Cunningham McLaughlin, "Introduction," xi*
It is commonly said that unconscious sources are the best; that is to say, not chronicles written with express purpose to hand down opinions and knowledge of events to succeeding generations, but materials prepared without reference to future times. ... [A]ny diary, especially one written faithfully for one's own eye without the future reader continually in mind, artificial though it be, is necessarily of great value in letting us see the man [or woman] that writes and in giving us a view of passing events as he [or she] sees them.Diaries and other records kept for personal recollection are preferable to those manufactured for posterity. Diaries written for revelation will contain deception, but there is less motivation to deceive oneself.
The Light of Limited Experience
President James K. Polk's diary is particularly useful because it "does not appear to have been written with the expectation that it would be conned by future historians" (xiii). Moreover, Polk was "peculiarly simple in his make-up" (xiii); he was not devious in his writing.
[H]e moved straight ahead with unusual directness, following his course unflinchingly, guided by the light of a limited experience and often led by a prejudice or a partisan antipathy which one can fairly easily detect.Polk, according to McLaughlin, is easy to read and shallow, and was thus incapable of guile.
By nature he was too simple, too plainly lacking in wide sympathy, too narrow in his emotions, too straightly hemmed in by education and practices of life, to become the prey of conflicting impulses.McLaughlin mentions other sources concerned with the period of Polk's presidency, including Thomas Hart Benton, Thirty Years' View; or, History of the Working of the American Government for Thirty Years, from 1820 to 1850 (1856). But we know what Benton could not know because Polk's diary reveals things about which "benton was sometimes in the dark or was but shrewdly guessing at what we know to be the fact" (xii).
We can see the Polk Administration more clearly than Benton because we have Polk's own account. We can understand Polk's actions better than the President himself because we see their effects, and because we are sophisticated enough to see through his prejudices.
*McLaughlin, Andrew Cunningham. "Introduction" to The Diary of James K. Polk During his Presidency, 1845 to 1849. Milo Milton Quaife, ed. Volume I. Chicago: A. C. McClurg & Co, 1910.