This passage comes from The Analects, Book II, "Wei Zheng": "You, shall I teach you what knowledge is? When you know something, recognize that you know it; when you do not know something, recognize that you do not know it. That is knowledge." The meaning is: Zhong You (Zi Lu, a disciple of Confucius), do you clearly understand what I have taught you? To know is to know, and not to know is not to know—this too is a form of wisdom. From Confucius's words, it can be seen that the "knowledge" in "that is knowledge" fundamentally differs from the ordinary sense of knowledge, which involves grasping specific information. It is a "knowledge" within the methodological realm of understanding the world, emphasizing a factual and truthful attitude, thus possessing greater depth and practical value. In connection with Confucius's related discussions, this teaching to Zhong You carries at least the following two implications.
First, it advocates maintaining an honest, modest, and cautious attitude in the process of knowing, opposing subjective conjecture and stubborn arbitrariness. According to The Analects, Book 9, "Zi Han," "Confucius was free from four things: no preconceptions, no arbitrary decisions, no obstinacy, and no egoism." Among these, "no preconceptions" and "no egoism" mean not making baseless guesses and not being self-righteous. Confucius himself said, "There may be those who act without knowing; I am not like this. To hear much, choose what is good and follow it; to see much and remember it—this is the second best kind of knowledge." (The Analects, Book 7, "Shu Er") Some people, not knowing themselves, still fabricate things. Confucius's attitude was to acknowledge his ignorance, then increase his seeing and hearing. He further proposed that after extensive observation and hearing, if one is still uncertain, one should suspend judgment rather than speak or act hastily. "Zi Zhang (a disciple of Confucius) asked about obtaining an official salary. Confucius said, 'Listen widely, set aside what is doubtful, and speak cautiously of the rest—then you will make few mistakes. Observe widely, set aside what is perilous, and act cautiously of the rest—then you will have few regrets. If your words have few mistakes and your actions few regrets, your salary will be secured.'" (The Analects, Book 2, "Wei Zheng")
Second, acknowledging the infinite nature of knowledge—that humans can never fully exhaust truth. The world's categorized knowledge is boundless; as a single individual, one can only know a portion, while the rest remains beyond grasp due to limited capability. Therefore, regarding what one does not know, one must never pretend to know it. Recognizing this, people can choose appropriate objects of study and exploration based on practical needs and their own subjective and objective conditions. Thus, Confucius's view does not indicate weakness or retreat before infinite knowledge; on the contrary, it reflects his grasp of the laws of cognition. The Analects, Book Eleven, records: "Ji Lu (a disciple) asked about serving ghosts and spirits. The Master said, 'If you cannot yet serve people, how can you serve ghosts?' Ji Lu said, 'May I dare to ask about death?' The Master said, 'If you do not yet understand life, how can you understand death?'" Here, Confucius refused to answer Zi Lu's questions about serving ghosts and spirits and about death. Confucius's disciples said, "The Master did not speak of strange phenomena, feats of strength, disorder, or spirits." (The Analects, Book Seven) He himself said, "Respect ghosts and spirits, but keep them at a distance." (The Analects, Book Six) That Confucius did not speak of "feats of strength" or "disorder" may have been his reluctance; as for death, strange phenomena, ghosts, and spirits, it is likely that he never included them within his scope of inquiry, so he had no choice but to set them aside. The cognitive attitude Confucius advocated—"to know what you know"—had a significant influence on later generations. It left behind a simple, honest, and prudent scholarly style that shunned superficial display.