Full Idea
Defining concepts psychologically, in terms of the nature of the human mind, makes everything subjective, and if we follow it through to the end, does away with truth.
Gist of Idea
Psychological accounts of concepts are subjective, and ultimately destroy truth
Source
Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], Intro)
Book Reference
Frege,Gottlob: 'The Foundations of Arithmetic (Austin)', ed/tr. Austin,J.L. [Blackwell 1980], p.-5
A Reaction
This is the reason for Frege's passionate opposition to psychological approaches to thought. The problem, though, is to give an account in which the fixity of truth connects to the fluctuations of mental life. How does it do that??