 (ii) Concepts and rules of inference. If skeptics do not assent, then how can they understand the terms philosophers use (M 8.337–332a)? Even more radically, how can they even think (PH 2.1–12)? This objection, which Sextus says is continually raised against the skeptics, proceeds on the assumption that possession of concepts involves the acceptance of assumptions. For example, in order to examine a given theory of proof, the skeptic must have a notion of what proofs are. This involves assumptions: for example, the assumption that a proof contains premises and a conclusion. Sextus’ response to this objection invokes the Epicurean and Stoic theories of preconceptions. Human beings are not born with reason (Frede 1994, 1996). The acquisition of reason is a nature-guided process of concept acquisition. At a given age, children have completed this process. They have become rational, which means that they can perceive and think in a conceptual way. Only now, they have rational impressions to which they can assent. The acquisition of preconceptions did not involve assent, simply because the child was not yet rational (Brittain 2005).