Indexicals appear to represent an exception to, and thus a challenge for, the understanding of natural language as the grammatical coding of logical propositions; they thus "raise interesting technical challenges for logicians seeking to provide formal models of correct reasoning in natural language." They are also studied in relation to fundamental issues in epistemology, self-consciousness, and metaphysics, for example asking whether indexical facts are facts that do not follow from the physical facts, and thus also form a link between philosophy of language and philosophy of mind.