A natural behavior is used to characterize by differential equation established on human observations, which is assumed to be on one particle or one field complied with reproducibility. However, the multilateral property of a particle P and the mathematical consistence determine that such an understanding is only local, not the whole reality on P, which leads to a central thesis for knowing the nature, i.e. how to establish a physical equation with a proper interpretation on a thing. As it is well-known, a thing consists of parts. Reviewing on observations, we classify them into two categories, i.e. out-observation and in-observation for discussion. The former is such an observation that the observer is out of the particle or the field P, which is in fact a macroscopic observation and its dynamic equation characterizes the coherent behavior of all parts in P, but the later is asked into the particle or the field by arranging observers simultaneously on different subparticles or subfields in P and respectively establishing physical equations, which are contradictory and given up in classical because there are not applicable conclusions on contradictory systems in mathematics.