When looking at someone's chart to determine their intelligence astrologers would look at both Mercury and the moon

Mercury is more left brain logic/reasoning stuff

The moon is more right brain intuitive/creative stuff. Emotional intelligence, common sense

For example:

I have Mercury exiled in Sag in the 12H and the moon domicile in Cancer in the 7H

I had average SAT scores in high school because my math score sucked but my reading score was above average

Then I just studied social work & psychology which are more lunar

If one or both planets are in poor condition in your chart that doesn't mean you're stupid

It just means it may require more effort for you

I can do math. I just hate it. My brain just doesn't work that way and that's ok

I had to study my ass off to test out of remedial math when I started school at community college

Then when I passed the test and got into the regular college level classes it was awful (math was actually easier at the university I ended up at)

I was good at science btw which you would think is more Mercury but it has lunar elements

The lunar part would be hypothesis, theory, and concluding findings

Mercury would be actually handling all the data

The sun is the common thread between these two planets btw

The moon is connected to it by the lunar cycle

Mercury is connected to it by only being able to be 28° away

In a sense, they're messengers of spirit (the sun). They allow our spirit to interact with the world