Galen's on Demonstration

In Galen's On Demonstration, he says that nature will usually act in such a way as to prevent void from forming. But there is another context in which he discussed the topic of void - and this complicates matters considerably. The Greek version of his work on demonstration is lost; we only know about it through later reports or Arabic texts like text by Razi. He was an important figure for many philosophers who were interested in medicine at the time.