Calligraphy is written on a plane, and the concept of two dimensions is believed to be understandable by everyone. The lines of calligraphy are three-dimensional through the use of center, hidden front and side front, combined with the shade of pen and ink, so it becomes a three-dimensional concept. Calligraphy writing and appreciation, like music, have inherent melody, urgency, ups and downs, climax, bedding and so on. And has the concept of time, so it is a four-dimensional concept. These four dimensions are all contained in simple black and white lines, so it is a kind of composite art. The artistic language of calligraphy is not expressed by the concrete image and superficial meaning of words, but by the internal rhythm, rhythm and texture of lines and space, which is very similar to music, so it is an abstract art.
The above is my superficial understanding. I don't know if I made it clear. I hope it helps you.