The lover learns at last that there is no person quite transparent and trustworthy, but every one has a devil in him that is capable of any crime in the long run.
 Henry David Thoreau, A Week on the Concord and Merrimack Rivers (1849). copy citation

Context

“He is a material which no tool of mine will work. A naked savage will fell an oak with a firebrand, and wear a hatchet out of a rock by friction, but I cannot hew the smallest chip out of the character of my Friend, either to beautify or deform it. The lover learns at last that there is no person quite transparent and trustworthy, but every one has a devil in him that is capable of any crime in the long run. Yet, as an Oriental philosopher has said, “Although Friendship between good men is interrupted, their principles remain unaltered. The stalk of the lotus may be broken, and the fibres remain connected.”” source