Attitude verbs fall in different categories depending on the kind of sentential complements which they can embed. In English, a verb like know takes both declarative and interrogative complements. By contrast, believe takes only declarative complements and wonder takes only interrogative complements. The present paper examines the hypothesis, originally put forward by Hintikka (1975), that the only verbs that can take both that -complements and whether -complements are the factive verbs. I argue that at least one half of the hypothesis is empirically correct, namely that all veridical attitude verbs taking that -complements take whether -complements. I distinguish veridical verbs from factive verbs, and present one way of deriving the generalization. Counterexamples to both directions of the factivity hypothesis are discussed, in particular the case of emotive factive verbs like regret , and the case of non-veridical verbs that licence whether complements, in particular tell, guess, decide and agree . Alternative accounts are discussed along the way, in particular Zuber (1982), Ginzburg (1995) and Saebø (2007).