Limits of Introspection
Watching one's own mind is an attractive way to study consciousness — what could be closer than direct perception? — but it has its pitfalls. For instance, consider the process of falling asleep. Various mental engines shut down or become disrupted and unreliable. As soon as the process that records new memories stops running you won't be able to recall anything that happens thereafter. As you lose consciousness you might consistently see flocks of pink eagles and perceive the taste of chocolate-coated newspapers — but you'd never remember it!