Silence is audible to all men, at all times, and in all places. She is when we hear inwardly, sound when we hear outwardly.
 Henry David Thoreau, A Week on the Concord and Merrimack Rivers (1849). copy citation

Context

““Let the immortal depth of your soul lead you, but earnestly extend your eyes upwards.” As the truest society approaches always nearer to solitude, so the most excellent speech finally falls into Silence. Silence is audible to all men, at all times, and in all places. She is when we hear inwardly, sound when we hear outwardly. Creation has not displaced her, but is her visible framework and foil. All sounds are her servants, and purveyors, proclaiming not only that their mistress is, but is a rare mistress, and earnestly to be sought after.” source