The primary tasks of the therapist can be described as listening to what the client says and making space for what the client has not yet said. According to Anderson and Goolishian, the therapist should take a not-knowing stance in this dialogic process. The question remains, however, what not-knowing exactly means. In this article, I will explore this question and I will propose the concept of constructive hypothesizing. Constructive hypothesizing is described as a process in which there is a movement back and forth between knowing and not knowing. Of central importance are creative and dialogic understanding, rather than knowledge. Recommendations are made to ensure the constructive and collaborative use of hypotheses in the therapeutic dialogue.