In ch. 2 of The Intellectual Life Sertillanges points out that “Purity of thought requires purity of soul.” He quotes St. Thomas of Aquin:
The exercise of moral virtues, of the virtues by which the passions are held in check, is of great importance for the acquisition of knowledge.
He then asks, “What are the enemies of knowledge?” He mentions “sloth, the grave of the best gifts”, “sensuality, which makes the body weak and lethargic, befogs the imagination, dulls the intelligence, scatters the memory”, pride, envy and irritation. It is these obstacles which prevent a man of study from reaching the level of his own gifts.
Great personal intuitions, piercing lights, are in men of equal powers the consequences of moral progress, of detachment from self and from the usual commonplace things, of humility, simplicity, discipline of the senses and the imagination, of an eager impulse towards the great ends.