The Importance of Valves in Demonstrations

In his work on demonstration, Galen says there are certain standards of proof that we need to meet in order to take ourselves to have demonstrated something. He was basically telling them off for not being sufficiently rigorous in their argumentation. The reason we know about this is from both the Greek and the Arabic tradition. It looks like at least a lot of this work was devoted to showing that other philosophers had given arguments which they thought were convincing but weren't demonstrative.