What is demonstrative reasoning?

PROOF (DEMONSTRATIVE REASONING): When the argument is valid AND the premises are self-evident (i.e., no rational person could doubt them), then the argument is a PROOF. DEMONSTRATIVE REASONING (PROOF) is a one-way street, because the reasoning goes in one direction only (from the Premises to the Conclusion).

Taken from: http://faculty.washington.edu/wtalbott/phil102/trweek1.htm