How do you define Ijmā' and tawātur?
If a generation knows something for a fact andthere is no dispute over it, we say that there is a consensus –Ijma'– over it.
If one generation conveys a fact or practice to the other and so on without interruption, we say that the fact or the practice has reached us through Tawatur or Uninterrupted Continuity. This communication occurs in such a manner that the number of communicators in each generation is so large that there is no possibility of the fact being misperceived, misconstrued, forgotten or any agreement or conspiracy to tell a lie and misinform the entire next generation, and therefore it does not remain necessary to investigate the character or memory of the narrators.
If consensus – Ijma' – occurs once over a fact or a practice in the historic times, it becomes an integral part of history and necessarily travels onward through Tawatur. If the consensus occurred in pre-historic times it is possible that the consensus fails to survive in later generations because it could be difficult for the consensus to perpetuate. The Ijma' is therefore sufficient to prove a fact beyond any shadow of doubt. A historical phenomenon would be even more authentic, if it is backed up by both Ijma' and Tawatur.