The mind travels from thought to thought randomly. We think in intervals that bounce from one memory to the next. Focus on one thought at a time is an act of actually looking at that thought. Focusing on anything is actually looking at the object clearly, the mind seeing the object and rationalizing that object. Random thoughts can be cleared through the practice of focus. Za Zen meditation teaches the mind to look at one thought at a time clearing the path for clear interpretation. Practice is the key. Sit and breath in the nose and out the mouth, count up from one to ten and back down again focusing only on the breath, in the nose and out the mouth. This is the beginning of Zen meditation.