What does "sich" do in this context?

Question

As an English speaker learning German, "sich" is a very mysterious word. Consider these sentences from the schlagen entry in Duden:

die Nachricht ist ihm auf den Magen geschlagen

die Erkältung hat sich ihm auf die Nieren geschlagen

Both sentences appear to me to have identical structure, except for the "sich". So, what is the function of "sich"? Is it simply to "permit" the use of haben instead of sein? Is there some subtle difference in meaning? Why would anyone want to bother with the variation? And why would "sich" be needed instead of simply using either haben or sein?

Answer 1

That idiomatic use of schlagen is very tricky.

My explanation is that the piece ist … geschlagen is technically static passive voice, and passive voice doesn't feature an accusative object. While hat geschlagen is Perfekt tense active voice and may feature an accusative object.

But it pretty much ends with that. There is no deeper logic why you would choose one or the other variant, or whether you should use sich with haben or leave it out, which is as correct.

Answer 2

The first example is normal passive voice. While the meaning is similar to the English ... hit him hard, German uses passive in connection with schlagen. Using hat ihn hart getroffen instead would also require the active voioce.

I would explain the second example as a personification of the sickness. One supposes, the cold has a will of its own and chooses to affect (in German: take a seat at) the kidneys. The cold afterwards sort of "resides there", therefore a reflexive pronoun applies (similar to: Ich habe mich auf den Stuhl gesetzt.). A slightly easier example would be:

Die Erkältung hat sich festgesetzt.