I've been learning idioms related to body parts, when some like these struck me:

1 A lot of young vandals who go looking for trouble are not right in the head.

2 Can you do this calculation in your head?

The in the head in 1 denotes a not-so-correct moral code in "young vandals"'s mind.

The in one's head (as in "in your head") in 2 denotes doing something inside your mind instead of physical acts.

My question is, as a simple difference between "the" and "one's" in an idiom can cause such diversion in meanings, is there any clear sign to tell when to choose the and when using one's instead? Or are the idioms' choice of words like these just as random as its origin and spread?



To be "right (or not right) in the head" is almost a set expression. "Messed up in the head" also works, but in general I can't think of other adjectives that can be combined with "in the head" in this way.

"In one's head" is a prepositional phrase which constrains the action of the sentence to the realm of mental thought. You described it pretty much correctly. Some other example usages are:

  • I solved the problem in my head.
  • I have a song stuck in my head.
  • I can't get that idea out of my head.
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  • So I guess there’s no rule? You will have to remember which is a prepositional phrase and which is a set expression, then? – L. S. Jeong Pótrekin Jun 24 '19 at 9:09
  • I think so, yes. Sadly this is very often the case with English; for example, phrasal verbs! You just have to learn them. As a German language learner I can totally sympathize, although German has its own unique challenges. – TypeIA Jun 24 '19 at 9:12
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    By the way, "in the head" is also a prepositional phrase - it's just that its usage is limited to certain set expressions, while "in one's head" has broader usage. That's the bit that you "just have to learn" I think. – TypeIA Jun 24 '19 at 9:14

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