From The Gates of Prayer, The New Union (Reform Jewish) Prayer Book:

Cherish your doubts, for doubt is the handmaiden of truth. Doubt is the key to the door of knowledge; it is the servant of discovery. A belief which may not be questioned binds us to error, for there is incompleteness and imperfection in every belief.

Doubt is the touchstone of truth; it is an acid which eats away the false.

Let none fear the truth, that doubt may consume it; for doubt is a testing of belief.

For truth, if it be truth, arises from each testing stronger, more secure. Those who would silence doubt are filled with fear; the house of their spirit is built on shifting sands.

But they that fear not doubt, and know its use, are founded on a rock.

They shall walk in the light of growing knowledge; the work of their hands shall endure.

Therefore, let us not fear doubt, but let us rejoice in its help: It is to the wise as a staff to the blind; doubt is the handmaiden of truth.