Frege vs. Idealism
In a nut shell Idealism is the position that IDEAS, not objects, are the basis of reality. So for idealists the meaning of an expression is the idea associated with that expression. For example, if one were to think of a lamp , then under the idea theory of reference the visual image that we pull up would be the meaning of the expression lamp . Frege rejects the idea theory of reference because he believes that ideas are subjective. That is to say that they are private mental images that have only one owner. He argues that sense is objective because many people do grasp the same thought and thoughts can be conveyed to others. If communication is possible then sense must be able to be grasped by others and the idea associated with an expression must be private. But remember that the idealist argues the meaning of an expression is the idea associated with it. So it follows that either language is impossible, or ideas are not the senses of our expressions. So for Frege, language is obviously possible, then it must be the idea theory of reference that is false. If ideas are private then the idealist would have to explain how language is possible since the ideas about an object are in ones head and no other person can have the same idea. So for Frege, language presupposes that there are intersubjective objects that we can talk about. Therefore, sense and reference are objective if language is possible.