"When you are familiar with this pen, you will see what you want to draw." What you want to draw does not represent the real thing, but the vivid image reflected on your chest. The "vision" mentioned here is to concentrate on thinking; The word "look" here refers to the meaning reflected in the mind.
"knowing why" means knowing how to do it in your chest; "Can't do it" means that it can't be done in practice. The two are not unified, that is, the heart and the hand cannot correspond.
When Su Shi talks about the theory of bamboo painting, he is actually praising Wen Heke, the summarizer and practitioner of this theory.
As a memorable prose, it mainly describes the communication between Su Shi and Wen He with bamboo as the topic and poetry as the joy, reflecting their sincere feelings. Taking bamboo as the theme of poetry, writing about Su Shi and taking poetry as pleasure in writing, embodies the simple but not vulgar character of Wen and Ke. Exposing this painting has aroused people's thinking about things and memories of the past, which expresses the author's condolences to his dead friend.