Receptive and attentive in every cell of the body, you learns to think with the body, unconsciously. During zazen, thoughts, conscious and subconscious, naturally and continuously rise to the surface of our mind. Don't try to stop these thoughts from arising. But at the same time, don't get involved with the thoughts or let them take you away from concentration on posture and breathing. Just let the thoughts pass, like clouds in the sky, neither opposing them nor attaching to them.
Shadows pass and vanish.
Images arise from the subconscious, then disappear. The brain becomes deeply calm. One arrives at the deep unconscious, beyond thought, to hishiryo consciousness, true purity. In Japanese, shiryo is thinking, fushiryo non-thinking. But hishiryo is absolute thinking, beyond thinking and non-thinking.