The Importance of Vapor in the Interstices

Galen showed that air is a body, he supported his claim that void does not exist in the interstices. But then how could he first claim that one cannot know whether void is existent or not? So here's what he's saying. Imagine a hypodermic needle, and imagine that the needle is blocked, okay? So you can't sort of spritz anything out of it. And the plunger is back, and there's nothing but air inside the needle, right? So you try to force the plunger in, and it won't go anywhere. Galen says, look, this proves that there's no void in the air"