  The general class of natural signs signifying unintentionally by their essence (1) is divided according to the relation between a sign and its significate into the three subclasses of (1.1) inferential signs based on a more or less constant concomitance of sign and significate, (1.2) iconic signs, based on similarity in appearance, and (1.3) signs based on a causal relation between the sign and the signified thing. The signs of inference (illatio) are subdivided into (1.1.1) necessary and (1.1.2) probable signs, both of which are further differentiated according to the three possible directions of temporal reference (present, past, future). Bacon gives to understand that he takes inferential and iconic signs to be signs more properly than the members of third class, i.e., signs based on a causal relation (later in the Compendium studii theologiae he will drop this class entirely). He justifies this by pointing to the fundamental difference between sign relations and causal relation: whereas sign relations are necessarily constituted by an interpreter, causal relations exist independently of any such one alone by reason of the order of  nature.[26]