I have a special definition for the word "Nature." To me, it is the set of laws which rules the universe.
From the birth of the little blade of grass to the explosion of a supernova, the rules governing the creation, the development, the decline and the destruction of all things are parts of Nature. From the unfoldment of the finest thought to the eruption of the most massive volcano, the same laws apply the same way upon material as well as subtle levels.
Therefore in order to live a harmonious life, we have to know the laws of Nature. Otherwise we may break them unwillingly and suffer without knowing why.