I came across the sentence 'He was a person with an inquiring turn of mind even from his early teens', and I found 'turn of mind' means 'one's temperament'. Why does that 'turn' have such a special meaning? I looked 'turn' over in many dictionaries but I couldn't find out why.
I'm not entirely sure, but:
(his, her, their) mind(s) is/are turning
is a idiotmatic expression meaning that someone is thinking.
A turn of mind, therefore refers to one's thinking process, one's thoughts and beliefs.
There is also an idiomatic expression "The wheels or cogs are turning" suggesting that one's mind has clock gears inside that make it work.