In the Logical Investigations, Edmund Husserl argues that a proper understanding of logic requires that one distinguish between the indicative function which signs, including linguistic signs, can have from the expressive function which only linguistic signs can have. This is important for a proper understanding of logic, since that discipline is concerned only with the meanings expressed by linguistic signs. While linguistic signs can exercise both functions, and indeed in communication they exercise both functions simultaneously, Husserl argues that even when both are present, the two functions can be distinguished from one another, and he thinks that in soliloquy we find the expressive function unaccompanied by the indicative function.