This quote is saying that there is always an exception. Details that are used to prove general ideas is where truth resides. When an idea is more specific it contains more meaning.
Holy Thursday (Songs of Experience) Is this a holy thing to see, In a rich and fruitful land, Babes reduced to misery, Fed with cold and usurous hand? Is that trembling cry a song? Can it be a song of joy? And so many children poor? It is a land of poverty! And their sun does never shine. And their fields are bleak & bare. And their ways are fill'd with thorns. It is eternal winter there. For where-e'er the sun does shine, And where-e'er the rain does fall: Babe can never hunger there, Nor poverty the mind appall.