I have heard many times in the name of Rabbi Shimshon Rafael Hirsch that every Hebrew shoresh (usually defined with three letters) has a more basic root of two letters. I also heard in his name that letters which are phonetically similar are also interchangeable in shorashim.
Is it true that he said this? If so, where did he say this? Are there any rules for the application of this?
(The concept of two-letter shorashim is not completely foreign to traditional Judaism; for example, the HaKsav VeHaKabalah uses it.)