When we read, the author thinks for us. We can read ourselves stupid if we just repeat the author's mental process. People who just let their own thoughts wander can be more creative than those who just read.
Book: Parerga and Paralipomena
From Arthur Schopenhauer in Parerga and Paralipomena Chapter 24 on reading and books:
"When we read someone else thinks for us: we merely repeat his mental process. This is like the pupil who in learning to write traces with his pen the strokes made in pencil by the teacher. Accordingly in reading we are for the most part absolved of the work of thinking."
"This is why we sense relief when we transition from preoccupation with our own thoughts to reading. But during reading our mind is really only the playground of the thoughts of others. What remains when these finally move on? "
"It stems from this that whoever reads very much and almost the whole day, but in between recovers by thoughtless pastime, gradually loses the ability to think on his own – as someone who always rides forgets in the end how to walk. But such is the case of many scholars: they have read themselves stupid. For constant reading immediately taken up again in every free moment is even more mentally paralysing than constant manual labour, since in the latter we can still muse about our own thoughts."
"But just as a coiled spring finally loses its elasticity through the sustained pressure of a foreign body, so too the mind through the constant force of other people's thoughts. And just as one ruins the stomach by too much food and so harms the entire body, so too we can overfill and choke the mind with too much mental food."
"For the more one reads, the fewer traces are left behind in the mind by what was read; it becomes like a tablet on which many things have been written over one another. Therefore we do not reach the point of rumination; but only through this do we assimilate what we have read, just as food does not nourish us through eating but through digestion."