What was understood to be his education was simply the practice of reading, writing, and spelling, carried on by an elaborate appliance of unintelligible ideas, and by much failure in the effort to learn by rote.
As for Tom’s school course, it went on with mill-like monotony, his mind continuing to move with a slow, half-stifled pulse in a medium uninteresting or unintelligible ideas.
There are no more uses of "intelligible" in the book.
Show samples from other sources
The Director wanted us to feel like we were there, but the dialogue was not intelligible.