James Mill's concept of mental physics describes the creation of a complex idea as:

James Mill's concept of mental physics emphasizes the way in which complex ideas are formed through the combination of simpler ideas, establishing a systematic relationship between them. In this framework, the process is described as orderly and predictable, rather than chaotic or random.

This notion suggests that the synthesis of complex ideas follows a structured, step-by-step method, where simpler elements interact in a manner that culminates in the emergence of more intricate concepts. By viewing the formation of complex ideas as a simple linear process, Mill underscores the cognitive mechanics involved in understanding how people blend their mental experiences to create richer, more nuanced thoughts.

The other options do not align with Mill's view: randomness does not reflect the systematic nature of thought processes he describes, emotional reactions do not encompass the rational methodology behind idea formation, and associative formulation, while somewhat related, does not capture the linearity and simplicity that Mill's theory emphasizes in his explanation of mental processes.