In most important respects, J. S. Mill accepted his father’s brand of associationism. J. S. Mill believed that (1) every sensation leaves in the mind an idea that resembles the sensation but is weaker in inten- sity ( J. S. Mill called ideas secondary mental states, sensations being primary); (2) similar ideas tend to excite one another (James Mill had reduced the law of similarity to the law of frequency, but J. S. Mill accepted it as a separate law); (3) when sensations or ideas are frequently experienced together, either simultaneously or successively, they become associ- ated (law of contiguity); (4) more vivid sensations or ideas form stronger associations than do less vivid ones; and (5) strength of association varies with fre- quency of occurrence. With only the minor excep- tion of the law of similarity, this list summarizes James Mill’s notion of “mental physics” or “mental mechanics.”
John Stuart took issue with his father on one important point, however. Instead of agreeing that complex ideas are always aggregates of simple ideas, he proposed a type of mental chemistry. He was impressed by the fact that chemicals often combine and produce something entirely different from the elements that made them up, such as when hydro- gen and oxygen combine to produce water. Also, Newton had shown that when all the colors of the spectrum were combined, white light was produced. J. S. Mill believed that the same kind of thing hap- pens in the mind. That is, it was possible for elemen- tary ideas to fuse and to produce an idea that was different from the elements that made it up.