  In sum, if all knowledge requires demonstration, and all demonstration proceeds from what is more intelligible by nature to what is less so, then either the process goes on indefinitely or it comes to a halt in undemonstrated first principles, which are known, and known securely. Aristotle dismisses the only remaining possibility, that demonstration might be circular, rather curtly, with the remark that this amounts to  ‘simply saying that something is the case if it is the case,’ by which device ‘it is easy to prove anything’ (APo. 72b32–73a6).