Mental chemistry, as proposed by John Stuart Mill, suggests that simple ideas combine to create:

John Stuart Mill's concept of mental chemistry posits that simple ideas, which can be understood as the foundational elements of thought, can combine in various ways to produce new and complex ideas. This process reflects the idea that the mind functions similarly to a chemical process where elemental substances combine to form new compounds with unique properties.

In this framework, the new complex ideas are not merely combinations of the original simple ideas but represent something greater than the sum of their parts. Mill's approach emphasizes creativity and the transformative nature of mental processes, suggesting a dynamic interplay between basic thoughts leading to more sophisticated and intricate concepts. This aligns closely with contemporary understandings of cognition, where novel thinking often emerges from the recombination of simpler pieces of information.

The other options do not capture the essence of Mill's theory. For instance, suggesting that the combination results in merely a sum ignores the novel outcomes that can arise when ideas merge. Illusionary concepts and only different variations do not encapsulate the profound psychological transformation that Mill described.