Ad Homien

Ad Ignorantiam

The appeal to feelings or prejudices rather than intellect.
Marked by or being an attack on an opponent's character rather than by an answer to the contentions made

The use of an unanswerable challenge to disprove rather than by serious attempt to prove.
Johnson: It is impractical to send more men to the moon because the money spent for that project could be spent on helping the poor..
Hanson: It is not impractical.
Johnson: Why?
Hanson: Just try to prove that I wrong.
(Hanson is defending his claim by an ad ignorantiam, i.e., his claim is true, if Johnson cannot refute him.)