Environmental historians distinguish between primary and secondary sources for reasons of utility. What you will quickly discover is that their definitions of primary and secondary sources are often intentionally arbitrary. What constitutes a primary or secondary source, in other words, often depends upon the nature of the research question being asked. We suggest that with every document you encounter, you ask yourself to think of questions that would shift the conception of a particular source from primary to secondary and vice versa. Doing so will expand your thinking about how research questions can shape the way you use certain documents as evidence.