How did Joseph Priestley’s views about facts and hypotheses shape his scientific work?

Joseph Priestley’s scientific work was fundamentally shaped by his preference for empirical "facts" over theoretical "hypotheses." Influenced by his Dissenting convictions, which held that dogma and prejudice were obstacles to individual inquiry, Priestley believed that scientific progress relied more on the accumulation of observable facts by any diligent researcher than on the speculative insights of a few geniuses.1

This methodological approach had several key impacts on his work:

• Experimental Focus: His preference for facts led him to prioritize the design of ingenious apparatuses, such as his improved pneumatic trough, which allowed him to isolate and study gases that were previously difficult to examine.2
• Opposition to Speculation: Priestley’s skepticism toward abstract theorizing fueled his resistance to the "new system of chemistry" proposed by Antoine Lavoisier. He viewed the imposition of such theories as akin to the religious and political dogmas he opposed, leading him to defend the phlogiston theory even as it was being superseded.3
• Practical Application: Consistent with his belief that science should support social progress, he applied his experimental findings toward practical ends, such as designing apparatuses for carbonated water and eudiometers for sanitary reform.2

Ultimately, Priestley advocated for a patient, humble, and experimental approach to nature, believing that science could best support piety and liberty when it avoided speculative theorizing.3