Full Idea
With the psychological conception of logic we lose the distinction between the grounds that justify a conviction and the causes that actually produce it.
Gist of Idea
Psychological logic can't distinguish justification from causes of a belief
Source
Gottlob Frege (Logic [1897] [1897])
Book Reference
Frege,Gottlob: 'Posthumous Writings', ed/tr. Hermes/Long/White etc [Blackwell 1979], p.147
A Reaction
Thus Frege kicked the causal theory of justification well into touch long before it had even been properly formulated. That is not to say that there is no psychological aspect to logic, because there is.