Begs the Question

I will try it again with another behaviour of my homemate:

• Amir is not lying when he speaks.
  


• Amir is speaking.
  


• Therefore, Amir is telling the truth

Although these statements have a logical form, they do nothing to convince one of the truthfulness of the speaker because the matter (that is, what the words actually symbolize) of the major premise (the general supposition Amir does not lie when he speaks) and the conclusion are actually the same thing. In seeking to prove Amir's truthfulness, the speaker asks his audience to assume that Amir is telling the truth, so this actually proves "If Amir is not lying, then Amir is telling the truth." Such arguments are formally logical. That is, the conclusion does formally follow; however, since it is materially identical to the major premise, the argument is said to be materially invalid. All self-circular arguments have this characteristic: that the proposition to be proved is assumed at some point in the argument.