Full Idea
With Associationism there proved to be no way to get a rational mental life to emerge from the sorts of causal relations among thoughts that the 'laws of association' recognised.
Clarification
Because it is purely mechanical
Gist of Idea
Hume's associationism offers no explanation at all of rational thought
Source
Jerry A. Fodor (Psychosemantics [1987], p. 18)
Book Reference
Fodor,Jerry A.: 'Psychosemantics' [MIT 1993], p.18
A Reaction
This might not be true if you add the concept of evolution, which has refined the associations to generate truth (which is vital for survival).