From the Oxford English:

recto
2 (honrado) honest, upright

Not really sure of this but there are loads of references to 'straight intelligence'.

e.g.
That could work in a setting with Amodern education system. If you want to use it in your game it's certainly a workable option. Another viable option, as mentioned above, is to just use straight intelligence checks for knowledge not covered by the presented skills.

However, the game is designed to take place in a medieval european type setting. In that setting a person would be much more likely to study a specific subject intensively than to have a well rounded modern education. Schools in the modern sense simply didn't exist. This is probably why the designers didn't include an "Education" skill. It didn't fit into the setting they were creating.

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Note added at 13 mins (2011-07-13 16:32:19 GMT)
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2 Jul 2010 – Give him a role that he CAN live up to… in fact, if you want to talk straight intelligence, he is brighter than most in that dressing room. ...

http://books.google.com/books?hl=de&id=wX0rRg0xZiMC&q=plain ...
"When I see a man like George Peabody - a man of plain intellect and moderate education - ..."

or the clear intelligence expressed in... Another interpretation.

Plain intellect sounds very good in this context.
However, the author may have been referring to
something more akin to the term, "common sense".

Or irreprochable may be. I don't think texts have intellect or intelligence though. Intelligent nature would work, but I think that this would be tantamount to the logic, reasoning or rationale of the texts.

HTH

Room for one more? Just another possibility.

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Note added at 3 days4 hrs (2011-07-16 20:48:23 GMT)
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Looking at the DRAE definitions for *recta* and *inteligencia*, how would "strict understanding" work here?

Recto/recta:
5. adj. Se dice del sentido primitivo o literal de las palabras, a diferencia del traslaticio o figurado

http://drae2.es/recto

inteligencia:
"Sentido en que se puede tomar una sentencia, un dicho o una expresión"

http://drae2.es/inteligencia