Gauss: "I mean the word proof not in the sense of the lawyers, who set two half proofs equal to a whole one, but in the sense of a mathematician, where ½ proof = 0, and it is demanded for proof that every doubt becomes impossible."
A standard to which Gauss himself was sometimes unable to adhere -- but he seemed to feel that Dirichlet always could and did, which is why he held that great master in such high esteem. Imagine being esteemed highly by Gauss!