  But in De Doctrina Christiana, after abandoning the skeptical position, Augustine redefines the sign accordingly, claiming that “a sign is something which, offering itself to the senses, conveys something other to the intellect (Signum … est res praeter speciem quam ingerit sensibus, aliud aliquid ex se faciens in cogitationem venire) (Augustine De doctr. chr. II 1, 1963, 33). In contrast to his former view, he is now attributing a fundamental epistemic function to the sign, claiming that “all instruction is either about things or about signs; but things are learnt by means of signs” (Omnis doctrina vel rerum est vel signorum, sed res per signa discuntur) (Augustine De doctr. chr. I 1, 1963, 9). The borderline between things and signs and thus the sign itself is defined functionally rather than ontologically: signs are things employed to signify something (res … quae ad significandum aliquid adhibentur) (Augustine De doctr. chr. I 1, 1963, 9). Augustine divides the sign into the two main classes of natural signs (signa naturalia) and given signs (signa data). “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”,[8]  as, for example, smoke when it indicates fire, the footprint of an animal passing by, or the countenance of an angry or sorrowful man. “Conventional signs, on the other hand, are those which living beings mutually exchange in order to show, as well as they can, the feelings of their minds, or their perceptions, or their  thoughts.”[9]  Whether and to what extent such an “intention to signify” (voluntas significandi) can be assumed in cases of animal sign communication Augustine leaves  open.[10]