The study of numeracy (5)
I believe the answer is:
maths
This might be a double definition.
'the study' is the first definition.
(maths is a kind of study)
'numeracy' is the second definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'of' is the link.
Can you help me to learn more?
(Other definitions for maths that I've seen before include "18 across subject" , "Numbers subject" , "Adding and subtraction, etc" , "Study of numbers (abbr.)" , "Arithmetic etc" .)