 That even one falsehood would be mistakenly treated as a genuine first principle – say, the belief that the senses are reliable, or that ancient authorities should be trusted – threatens to spread falsehood to other beliefs in the system. A collective doubt helps avoid such mistakes. It ensures that the method only approves candidate first principles that are unshakable in their own right: it rules out that the appearance of unshakability is owed to logical relations with other principles, themselves not subjected to doubt.