Very recently, it was shown that the class of languages with interactive proofs, IP, is exactly the class PSPACE. This surprising result elegantly places IP in the standard classification of feasible computations. Furthermore, the IP = PSPACE result reveals some very interesting and unsuspected properties of mathematical proofs. In this column, we define the width of a proof in a formal system $\cal F$ and show that it is an intuitively satisfying and robust definition. Then, using the IP = PSPACE result, it is seen that the width of a proof (as opposed to the length) determines how quickly one can give overwhelming evidence that a theorem is provable without showing the full proof.