Education...is a companion which no misfortunes can depress, no clime destroy, no enemy alienate, no despotism enslave: at home a friend, abroad an introduction, in solitude a solace, in society an ornament: it chastens vice, it guides virtue, it gives at once a grace and government to genius. Without it, what is man? A splendid slave, a reasoning savage.
Source, in full
Though sometimes attributed to Addison, this actually comes from a speech delivered by the Irish lawyer Charles Phillips in 1817, in the case of O'Mullan v. M'Korkill, published in Irish Eloquence: The Speeches of the Celebrated Irish Orators (1834) pp. 91-92.