"Book is painting for the heart" means trying to figure it out with your heart and expressing it with your heart. As for the quality of character and conduct, it cannot be fully presented. "Calligraphy is painting for the heart" reveals an important feature of calligraphy art, that is, the essence of art: calligraphy is the product of calligrapher's subjective spirit and the result of heart action.
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"Painting with books as the heart" occupies an important position in calligraphy theory. I read everyone's messages, and most of them are agree and disagree. Now let me express my opinion.
This formulation was first put forward by Chang Xiong of the Western Han Dynasty, and its original intention was not to refer to calligraphy, but to books (texts). Later, it was quoted to calligraphy by calligraphy theorists, and its influence on later calligraphy was manifested in the understanding and practice of calligraphy, which was a classic judgment.
Beginners are not familiar with the "book for the heart painting", which can be understood like this. If you want to write well, you must first have this word in your mind and start writing confidently before you can write well. With an image in your heart, you don't panic when you write, but you will naturally show it with a pen.
What calligraphy wants to express is mainly the calligrapher's personality style and psychological aesthetic orientation. However, this style and aesthetics can only be achieved by constantly learning from the predecessors and experiencing it with heart. If you write exactly like your predecessors and have no own style, you can only be called a "calligrapher."