In our example, the manifestations Heng Wu and Heng Yu show the same principle of Tao. In the fourth verse, Lao-tzu emphasizes that these two manifestations “appear at the same time and describe the same reality (Tao) with different names兩者同出、異名同謂.”
This is a critical statement in the logic structure of Tao philosophy. The manifestations are “two and one” at the same time. If we look at the nature of the myriad things in terms of Wu and Yu, we are dualistic and the views are unrealistic. If we look at the nature in terms of Heng Wu and Heng Yu, the two views may have different characteristics but represent the same reality. They represent the same principle of Tao. We may say that the manifestations are ontologically equivalent. 
Traditionally, there have been ambiguities in the interpretation of the “two” in this verse. In our model, the number “Two” should refer to Heng Wu and Heng Yu, since only these two have wholeness to be realities. The “two” should not be Wu and Yu, since Wu and Yu lack the wholeness to represent any reality of Tao.
 Professor Lik-kuen Tong uses the term “Ontological Equivalence” in his description of Tao philosophy. (Tong 2001) Such relation appears in the Western philosophy with various names, e.g., the Principle of Ontological Parity.