It is easier to write ten volumes of philosophy than to put one principle into practice.

Leo Tolstoy (1828-1910) Russian novelist and moral philosopher

Diary (1847-03-17)
    (Source)

From his earliest diary entry, when he was 18.Variants:

• "It is easier to produce ten volumes of philosophical writing than to put one principle into practice."
• "It is easier to write ten volumes on theoretical principles than to put one principle into practice."