"Which is Outside" is a series of paintings that Liu Kuo-sung began work on after his return to Taiwan in September 1967. Through the placing of a square space above the array of broad ink brush strokes, the painting seeks to achieve separation from existing space and create a separate spatial/temporal dimension. The space within the square is completely different from the bold brush strokes in the lower part of the canvas; it is as though one were looking out through a window into a separate space. But where is the window? And where is the space outside the window? The work creates a fascinating formal conundrum. The Fine Art Historian Wang Bomin described the painting in these terms: "It is another eye within outer space."

Liu Kuo-sung's broad brush stroke technique involves the use of large brushes on coarse, highly textured "Liu Kuo-sung Paper." The lines are thick, rugged and vigorous, embodying immense power. The rough texture of the paper creates a pattern of white space like a winding silken thread; the lively, breeze-blown image that this presents contrasts starkly with the thick blue-black brush strokes, adding an additional rhythmic beauty and sense of movement to the work. Within the square "window" there is another bold brush stroke, this time a green one. It is as though verdant mountains in the far distance were being squeezed by the rectangular frame. At the same time, the strong brush strokes in the lower part of the canvas spread out unconstrained in all directions; the contrast between the upper and lower portions of the painting adds to the dramatic tension and interest of the work.

The "Which is Outside" series marked a watershed in the evolution of Liu Kuo-sung's artistic style. The relatively small number of paintings in this series further enhances the importance and value of each individual painting.