One may know what another does not, but the utmost kindness cannot impart what is requisite to make the advice useful.
 Henry David Thoreau, A Week on the Concord and Merrimack Rivers (1849). copy citation

Context

“as the rainbow, however beautiful and unerring a sign, does not promise fair weather forever, but only for a season. I have known two or three persons pretty well, and yet I have never known advice to be of use but in trivial and transient matters. One may know what another does not, but the utmost kindness cannot impart what is requisite to make the advice useful. We must accept or refuse one another as we are. I could tame a hyena more easily than my Friend. 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.” source