The head's study (10)
I believe the answer is:
phrenology
This could be a double definition.
'the head's' is the first definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'study' is the second definition.
(phrenology is a kind of study)
Can you help me to learn more?
(Other definitions for phrenology that I've seen before include "Fanciful science" , "pseudoscience" , "Study of cranial bumps" .)