I know of an illiterate fisherman who built some of the best boats on the East Coast, by hand and without any formal mathematical calculations. Limited literacy and numeracy skills, but a great thinker and doer.

Oral societies, for instance, had no literacy but produced some great thinkers. I used to think that literacy was necessary for everyone, but I'm beginning to have my doubts. Mark Federman's essay on "Why Johnny and Janey Can't Read" [Google it] is worth a peek (or listen to the podcast):

"But orality has not structured society since ancient Greece, and literacy no longer structures society today. The challenge for all the Mr. and Ms. Smiths throughout the academy, and eventually in the secondary and primary classrooms throughout the world, is to recognize that the exclusive focus and predominance given to the pedagogical artefacts of a literate world is inconsistent with the skills necessary to participate in the discovery and production of
knowledge in a ubiquitously connected and pervasively proximate world."