Frege claims that it is “the business of the logician to conduct an unceasing struggle against psychology and in part against language and grammar as far as they do not express only what is logical” (NS, 7/PW, 6–7). The motives for a struggle against psychology are explained by the first principle; but why a struggle against language and grammar? We have to start with Frege's view that thoughts are expressed by sentences and, therefore, by means of a language. Language is not just one possible way of expressing a thought among others. We are able to think only by using a language, because “we are accustomed to think in some language” (NS, 6/PW, 6) and because “we have to use sensible signs to think” (SJCN, 48/83). Frege, following Trendelenburg's essay “Über Leibnizens Entwurf einer allgemeinen Charakteristik”, points out that “without the great invention of signs” our experience would be reduced to our given sense impressions and “conceptual thinking” would be impossible (ibid.). It is only through the use of signs that belong to some language or other that human beings are able to form the nonsensible faculty of thinking. Whereas the senses, taken by themselves, deliver mere ideas, the use of a language allows us to grasp thoughts.

Given this general view about the connection between thinking and using a language, it is obvious that the “struggle” against language cannot be decided in the way the struggle against psychology was by the first principle. Thinking is independent of having ideas and depends on using a language.