For him who has conquered the mind, the mind is the best of friends; but for one who has failed to do so, his mind will remain the greatest enemy. . .

Doubts and fears should be rigorously excluded; they are disintegrating elements which break up the straight line of effort, rendering it crooked, ineffectual, useless.  Thoughts of doubt and fear never accomplished anything and Never can.  They always lead to failure.  Purpose, energy, power-to-do, and all strong thoughts cease when doubt and fear creep in.   

- As A Man Thinketh, James Allen