All deep things are Song. It seems somehow the very central essence of us, Song; as if all the rest were but wrappages and hulls! The primal element of us; of us, and of all things. The Greeks fabled of Sphere-Harmonies: it was the feeling they had of the inner-structure of Nature that the soul of all her voices and utterances was perfect music. Poetry, therefore, we will call musical Thought. The poet is he who thinks in that manner. At bottom, it turns still on the power of intellect; it is man's sincerity and depth of vision that makes him Poet. See deep enough, and you see musically; the heart of Nature being everywhere music, if you can only reach it.
Thomas Carlyle (1795 - 1881)
Source: On Heroes, Hero-Worship and the Heroic in History, pp.83-85
As a single footstep will not make a path on the earth, so a single thought will not make a pathway in the mind. To make a deep physical path, we walk again and again. To make a deep mental path, we must think over and over the kind of thoughts we wish to dominate our lives.