Original sentence: (Proponents of the belief that Edward IV was illegitimate counter this by arguing that Edward was not the rightful king and therefore an attainder enacted by him had no validity.Numerous counter-arguments to this theory have been raised:.This line’s claim to the Crown is based squarely upon the argument--that  Edward IV  was not sired by Richard Plantagenet, 3rd Duke of York, and thus had no legitimate claim to the Crown
Edited sentence: However, this claim is invalid for numerous reasons:.This line’s claim to the Crown is based squarely upon the assumption--with debatable evidence--that  Edward IV  was not sired by Richard Plantagenet, 3rd Duke of York, and thus had no legitimate claim to the Crown