Mark the music.

77. For do but note a wild and wanton herd,
78. Or race of youthful and unhandled colts,
79. Fetching mad bounds, bellowing and neighing loud,
80. Which is the hot condition of their blood;
81. If they but hear perchance a trumpet sound,
82. Or any air of music touch their ears,
83. You shall perceive them make a mutual stand,
84. Their savage eyes turn'd to a modest gaze
85. By the sweet power of music: therefore the poet
86. Did feign that Orpheus drew trees, stones and floods;
87. Since nought so stockish, hard and full of rage,
88. But music for the time doth change his nature.
89. The man that hath no music in himself,
90. Nor is not moved with concord of sweet sounds,
91. Is fit for treasons, stratagems and spoils;
92. The motions of his spirit are dull as night
93. And his affections dark as Erebus:
94. Let no such man be trusted. Mark the music.

The Merchant of Venice – Act 5, Scene 1. Lines: 77-95.