A man of one book

Timeo hominem unius libri.

"I fear a man of one book."

The adage is attributed to Thomas Aquinas.  It has generally been used to disparage one whose learning was so narrow that he lacks the corrective of other points of view, other perspectives.  In its original use by Aquinas, however, it expressed the formidable opponent in a debate that is presented by someone who has specialized, when the debate is on his turf.  He is said to have recommended the deep study of a single book as the surest method to gain a good education.

Deep versus broad - the tension inherent in every autodidact's education.  For myself I like to combine both.  Broad surveys of the terrain, but when I find something that seems promising, I like to let it "possess" me as an obsession for a time.