The Bright and the Dark, these two paths are considered to be eternal paths in this world. By the one, one goes never to return, by the other, one goes to come back.
शुक्लकृष्णे गती ह्येते जगतः शाश्वते मते। एकया यात्यनावृत्तिमन्ययाऽऽवर्तते पुनः॥