Reason and consensus are both among the four sources of jurisprudence. But reason tells us not to believe in anything just because others believe in it without independent evidence. Why is then consensus also a source of jurisprudence?
Consensus (Ijmaa’إجماع) means when all the scholars agree on a matter, it means that Imam Al-Mahdi (AS) is approving their consensus, otherwise he should have informed any scholars otherwise. Consensus is an evidence because it reflects the approval of Imam Al-Mahdi (AS) on this matter of consensus.
Wassalam.