The second of four Relatively Adequate Hypotheses in World Hypotheses: A Study in Evidence (1942), _Chapter IX: Mechanism_ has a root metaphor of a machine (with variants in mechanical theory and electrical theory) that 20th and 21st century educated laymen readily understand.
_Mechanism_ can be compared to other Relatively Adequate Hypotheses in Root Metaphor, Categories, Theory of Truth.
Late in the chapter, Pepper described issues of inadequacy in the gap between (i) primary, effective qualities, and (ii) secondary, ineffective qualities.