This phrase comes from Immanuel Kant's 1764 essay, Observations on the Feeling of the Beautiful and Sublime. In this work, Kant (an enlightenment philosopher) explores these two emotions we feel when looking at things like mountains or roses or breathtaking artwork. Dostoevsky appropriates the term and uses it to describe, somewhat satirically, the Underground Man's "appreciation" for all that is fine in the world. The Underground Man tells us of his "attacks" of the sublime and beautiful, moments when he is "conscious of goodness" in the world. Unfortunately, those moments end up driving him into mire and filth. The sublime and beautiful, then, come to represent one half of the Underground Man's crazy flip-flopping between a life underground and an aesthete's awe of the best the world has to offer.