Incapable
Part of speech: adjective
Definition: Not being capable (of doing something); unable.
Example sentence: Those who occupy their minds with small matters, generally become incapable of greatness.
Incompetent
Part of speech: noun
Definition: A person who is incompetent.
Part of speech: adjective
Definition: Unskilled, lacking normally expected degree of ability.Unable to make rational decisions, insane or otherwise cognitively impaired.
Example sentence: I start out with the assumption that a lawyer in a criminal case is going to be incompetent - substantially so. I find my assumption to be rarely wrong. Yet society starts out with the very opposite assumption.