Step 1: Understanding the context of the philosopher.
In the passage, language is described as being used by a philosopher to clarify his ideas. The philosopher uses language not as an emotional outlet, but as a tool for thinking and presenting complex ideas.
Step 2: Function of language for the philosopher.
The philosopher uses language to express thoughts clearly, organize concepts, and communicate abstract ideas in a way that can be understood by others. It becomes an instrument of thought, aiding in the process of reasoning.
Step 3: Conclusion.
Thus, a philosopher uses language as a medium for thinking and clarifying ideas, making it an essential tool for intellectual communication.
Final Answer: \[ \boxed{\text{A philosopher uses language as an instrument of thought to clarify and communicate complex ideas.}} \]