One naturally expects Mathesis universalis and proof theory to have something to say about the concepts of proof and ground, but for the most part both are disappointing in that respect. Bolzano is however an exception ­– ground is a key concept in his Wissenschaftslehre and is closely connected with ideas that he has about ideal proofs. They resemble in some respects normal proofs in natural deduction. In the light of contemporary proof theory I try to develop Bolzano’s notion of ground to see if it can be used to clarify the concept of proof.