The 'Will'...

Failure comes when will surrenders to reason.

Bowl of Saki, by Hazrat Inayat Khan
Commentary by Pir-o-Murshid Inayat Khan:

Will is not [merely] 'a power', but it is all the power there is. How did God create the world? By will. Therefore, what we call 'will power' in us is in reality 'God power,' a power that increases by our recognizing its potentiality and proves to be the greatest phenomenon in life. If there is any secret behind the mystery of the world of phenomena that can be learned, it is will power. It is by will power that all we do, physically or mentally, is accomplished. Our hands, with all their perfect mechanisms, cannot hold a glass of water if there is no will power to support them. If will power fails, a person seemingly healthy will not be able to stand.

It is man's lack of faith that generally causes failures. In faith is the secret of fulfillment or non-fulfillment of every thought. There is no doubt about the fulfillment of a desire if man's faith works with it. But when one's own reason and doubt come and destroy the hope, one generally meets with failure. ... there are numberless people who are thinking and reasoning all their lives, asking themselves, 'Shall I do this? How can I do that? How can I overcome these obstacles? And all the time they are thinking of the hindrances, or waiting for suitable circumstances to arise, and they never do. Their whole life may be spent in the pursuit of something which reason prevents them from attaining.

An important rule of psychology is that every motive that takes its root in the mind must be watered and reared until its full development. And if one neglects this duty, one does not only harm the motive, but by this the will power becomes less, and the working of the mind becomes disorderly. Even if the motive be small and unimportant, yet a steady pursuit after its attainment trains the mind, strengthens the will, and keeps the inner mechanism in order.

For instance, when a person tries to unravel a knot, and then he thinks, 'No use giving time to it,' he loses an opportunity of strengthening the will and attaining the object desired. However small a thing may appear to be, when once handled, one must accomplish it, not for the thing itself, but for what benefit it gives.