Full Idea
It is the quest for system and simplicity that has kept driving the scientist to posit further entities as values of his variables. By positing molecules, Boyles' law of gases could be assimilated into a general theory of bodies in motion.
Gist of Idea
The quest for simplicity drove scientists to posit new entities, such as molecules in gases
Source
Willard Quine (On Multiplying Entities [1974], p.262)
Book Reference
Quine,Willard: 'Ways of Paradox and other essays' [Harvard 1976], p.262
A Reaction
Interesting that a desire for simplicity might lead to multiplications of entities. In fact, I presume molecules had been proposed elsewhere in science, and were adopted in gas-theory because they were thought to exist, not because simplicity is nice.