Man, though he have great variety of thoughts, and such from which others as well as himself might receive profit and delight; yet they are all within his own breast, invisible and hidden from others, nor can of themselves be made to appear.
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“which, being conversant about propositions, and those most commonly universal ones, has greater connexion with words than perhaps is suspected. These considerations, therefore, shall be the matter of the following chapters. CHAPTER II. OF THE SIGNIFICATION OF WORDS. 1. Words are sensible Signs, necessary for Communication of Ideas. Man, though he have great variety of thoughts, and such from which others as well as himself might receive profit and delight; yet they are all within his own breast, invisible and hidden from others, nor can of themselves be made to appear. The comfort and advantage of society not being to be had without communication of thoughts, it was necessary that man should find out some external sensible signs, whereof those invisible ideas, which his thoughts are made up of, might be made known to others.” source