Chapter 1.—Signs, Their Nature and Variety.
1. As when I was writing about things, I introduced the subject with a warning against attending to anything but what they are in themselves, 1758 even though they are signs of something else, so now, when I come in its turn to discuss the subject of signs, I lay down this direction, not to attend to what they are in themselves, but to the fact that they are signs, that is, to what they signify. For a sign is a thing which, over and above the impression it makes on the senses, causes something else to come into the mind as a consequence of itself: as when we see a footprint, we conclude that an animal whose footprint this is has passed by; and when we see smoke, we know that there is fire beneath; and when we hear the voice of a living man, we think of the feeling in his mind; and when the trumpet sounds, soldiers know that they are to advance or retreat, or do whatever else the state of the battle requires.
2. Now some signs are natural, others conventional. Natural signs are those which, apart from any intention or desire of using them as signs, do yet lead to the knowledge of something else, as, for example, smoke when it indicates fire. For it is not from any intention of making it a sign that it is so, but through attention to experience we come to know that fire is beneath, even when nothing but smoke can be seen. And the footprint of an animal passing by belongs to this class of signs. And the countenance of an angry or sorrowful man indicates the feeling in his mind, independently of his will: and in the same way every other emotion of the mind is betrayed by the tell-tale countenance, even p. 536 though we do nothing with the intention of making it known. This class of signs, however, it is no part of my design to discuss at present. But as it comes under this division of the subject, I could not altogether pass it over. It will be enough to have noticed it thus far.
See Book i. 519.