Thomas a Kempis, The Imitation of Christ Ch 3

"Happy is he whom truth by itself doth teach, not by figures and words that pass away; but as it is in itself (Psalm 94:12).

Our own opinion and our own sense do often deceive us, and they discern but little...It is folly to neglect the things that are profitable and necessary, and give our minds to that which is curious and hurtful: we have eyes and see not.

...Who hinders and troubles thee more than the unfortified affections of thine own heart?

...This ought to be our endeavor, to conquer ourselves, and daily wax stronger and to make a further growth in holiness. All perfection in this life hath some imperfection mixed with it; and no knowledge of ours is without some darkness."