Knowledge is certainly one of the means of pleasure, as is confessed by the natural desire which every mind feels of increasing its ideas.
 Samuel Johnson, The History of Rasselas, Prince of Abissinia (1759). copy citation

Context

“Are those nations happier than we? ” “ There is so much infelicity, ” said the poet, “ in the world, that scarce any man has leisure from his own distresses to estimate the comparative happiness of others. Knowledge is certainly one of the means of pleasure, as is confessed by the natural desire which every mind feels of increasing its ideas. Ignorance is mere privation, by which nothing can be produced; it is a vacuity in which the soul sits motionless and torpid for want of attraction, and, without knowing why, we always rejoice when we learn, and grieve when we forget.” source