proof by accumulated evidence: Long and diligent search has not revealed a counterexample.
proof by cosmology: The negation of the proposition is unimaginable or meaningless. Popular for proofs of the existence of God.
proof by mutual reference: In reference A, Theorem 5 is said to follow from Theorem 3 in reference B, which is shown to follow from Corollary 6.2 in reference C, which is an easy consequence of Theorem 5 in reference A.
proof by metaproof: A method is given to construct the desired proof. The correctness of the method is proved by any of these techniques.
