3 ideas
Klee defines the line as the path a point leaves when it moves, and the plane as the path a moving line leaves. Form is therefore recorded movement, not a static outline. Each dimension is produced by moving the element of the dimension below it, so a picture can be read as a record of the process that made it.
Klee sorts lines by how they relate to the planes they create. An active line moves freely and generates form. A medial line both draws a boundary and encloses a plane. A passive line only results from planes meeting or being coloured in. Seeing which role a line plays tells you whether the line or the plane is driving the composition.
Klee treats pictorial balance as a dynamic state that you check against gravity and a vertical axis, like a scale or a tightrope walker. A heavy element can be offset by a lighter one set farther out, or by a different quality such as colour weight or direction. Balance comes from forces answering each other, not from symmetry.