If Duì has appeared in your reading, the book is naming the doubled trigram of lake — joy above and below, communication between equally receptive surfaces. The character carries the sense of delight, of the open mouth in conversation, of pleasure that is shared. Lakes joined to lakes: the water reaches the same level on both sides, and the meeting is easy.

Classical commentary reads this hexagram as the configuration of authentic exchange. The joy here is not the joy of solitary fulfilment; it is the joy that arises in honest communication, in shared work, in the pleasure of two minds meeting in the same actual subject. The hexagram appears when the reader is at, or near, such an exchange — and is being asked to recognise it for what it is.

What the book counsels is captured in the image's instruction: 朋友講習 — friends discuss and practise. The reader is being pointed toward the kind of relationship in which talking and doing are interwoven, in which the conversation is part of the practice and the practice deepens the conversation. This is not casual sociability. It is the structured friendship of people who are working at something together.

Duì's failure mode is the performance of joy where the real exchange is missing. The book is alert to it. The hexagram appears when the reader is in proximity to real communicative joy and the temptation is to settle for its surface — the cheerful tone without the actual meeting. The clearance attaches to the real version. Keep the talking and the practising joined; the joy follows.