“Although these points of view do reflect the prevailing empirical climate of our times, they fail to express more than a small part of the value of personal documents for social science. Properly used, such documents anchor a discipline in the bedrock of human experience, make the most of the predilective value of the single case in the normal process of human thought, exploit the idiographic principles of reasoning, and aid in meeting (more adequately than can unaided actuarial methods of work) the three critical tests of science: understanding, prediction, and control.”
Allport, G. W. (1942). The use of personal documents in psychological science. Social Science Research Council.