This study investigates how human-AI collaboration supports Knowledge Building (KB), a process of collective idea advancement.We are trying to conceptualize whether it is possible that generative AI can serve as a collaborator in knowledge creation.Using discourse analysis of one human-AI dialogue, we identified 66 idea units across 14 turns of text discourse, including one Rise Above where the participant synthesized ChatGPT 5's ideas.While ChatGPT, as a collaborator, primarily restates human ideas, its elaborations can still catalyze the participants' higher-level reasoning.The findings indicate that KB can occur in human-AI dialogue, highlighting the need to design more inquiry-driven and epistemically engaged AI for collaborative knowledge advancement.