“How naive man is when he thinks he can measure the depth of the sea, the flow of a river, the speed of the wind, the distance between stars and galaxies. Is he still able to weigh his own foolishness and arrogance? That he can uncover the secrets behind all truths?”
Related quotes

River Man
Song lyrics, Five Leaves Left (1969)
Variant: Going to see the river man.
Going to tell him all I can
About the ban
On feeling free.If he tells me all he knows
About the way his river flows,
I don't suppose
It's meant for me.

As translated by Ejvind Haas
Siddhartha (1922)
Context: When you throw a rock into the water, it will speed on the fastest course to the bottom of the water. This is how it is when Siddhartha has a goal, a resolution. Siddhartha does nothing, he waits, he thinks, he fasts, but he passes through the things of the world like a rock through water, without doing anything, without stirring; he is drawn, he lets himself fall. His goal attracts him, because he doesn't let anything enter his soul which might oppose the goal. This is what Siddhartha has learned among the Samanas. This is what fools call magic and of which they think it would be effected by means of the daemons. Nothing is effected by daemons, there are no daemons. Everyone can perform magic, everyone can reach his goals, if he is able to think, if he is able to wait, if he is able to fast.

1830s, The American Scholar http://www.emersoncentral.com/amscholar.htm (1837)

Channing, reported in Bartlett's Familiar Quotations, 10th ed. (1919)

The Unity of Religious Ideals, Part I : Seeking for the Ideal.
The Spiritual Message of Hazrat Inayat Khan