Zen Master Dogen instructed,

The most vital concern in learning the Way is to practice zazen. In China, many people attained the Way entirely through the power of zazen. If one concentrates on practicing zazen continuously, even an ignorant person, who does not understand a single question, can be superior to an intelligent person who has been studying for a long time. Therefore, practitioners must practice shikantaza wholeheartedly without bothering to concern themselves with other things. The Way of the buddhas and patriarchs is nothing but zazen. Do not pursue anything else.  

At the time, Ejo asked, “In learning both sitting and reading, when I read the collections of the old masters’ sayings or koans, I can understand one thing out of a hundred or a thousand words, though I have never had such an experience in zazen. Should we still prefer to practice zazen?”  

Dogen replied, “Even if you may seem to have some understanding while reading koans, such studies will lead you astray from the Way of the buddhas and patriarchs. To spend your time sitting upright with nothing to be gained and nothing to be realized is the Way of the patriarchs. Although the ancient masters encouraged both reading and shikan zazen, they promoted sitting wholeheartedly. Although there are some who have gained enlightenment hearing stories of the masters, the attainment of enlightenment is due to the merit of sitting. True merit depends on sitting.” (Shobogenzo Zuimonki, Book 5, Chapter 23)