"[The distinction] fits some disciplines better than others: it works very well for history, in which primary sources are materials directly connected to a historical event or moment. . . . But it works less well for, say, philosophy, chemistry, or nursing" (67).
"[The distinction] is not absolute but relative to a researcher's project. In most instances, an article in a scholarly journal would generally be considered a secondary source. But it would become a primary source if your research problem concerned its author or the field itself: if, for example, you are writing the author's biography or trying to figure out whether patriotic historians have distorted stories of the Alamo" (67-68).
Booth, Wayne C., et al. The Craft of Research. 4th ed., U of Chicago P, 2016
A "rigid classification of primary or secondary sources can distort and misrepresent. It is impossible to assign a label to a source without knowing how it is used as evidence. The nature of a source does not derive from the kind of object it is, but rather from the purpose it serves in a historical investigation" (164).
Scheuler, Steven. "Primary and Secondary Sources in History: A Primer for Undergraduates, Challenges for Librarians." The Reference Librarian, vol. 55, no. 2, pp. 163-167, doi:10.1080/02763877.2014.881274
"For a thesis on 'Adam Smth's Economic Thought,' the object is Adam Smith's bibliography [i.e. his written works], and the instruments are other books on Adam Smith. In this case, we can say that Adam Smith's writings constitute the primary sources and the writings about Adam Smith are the secondary sources. . . . Naturally, if the topic were 'The Sources of Adam Smith's Economic Thought,' the primary sources would then be the books or other writings that inspired Adam Smith" (45).
Eco, Umberto. How to Write a Thesis. MIT P, 2015