lost in thought vs soul-searching

lost in thought

adj
  • Being preoccupied with one's thoughts rather than paying attention to reality. 

soul-searching

adj
  • Involving probing introspection, or a critical consideration of one's conscience, especially motives and values. 

noun
  • Probing introspection; a critical consideration of one's conscience; also, an instance of such consideration. 

How often have the words lost in thought and soul-searching occurred in a corpus of books? (source: Google Ngram Viewer )