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 telltale countenance, even 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.