incapable vs unable

incapable

adj
  • Not capable (of doing something); unable. 

  • Not in a state to receive; not receptive; not susceptible; not able to admit. 

unable

adj
  • Not able; lacking a certain ability. 

verb
  • To render unable; to disable. 

intj
  • Indicating that a requested course of action is not possible to carry out. 

How often have the words incapable and unable occurred in a corpus of books? (source: Google Ngram Viewer )