Now that we know what makes something a demonstration, we need to know what the first principles of a demonstration are. Thisis the burden of Book Two, Chapter Eight below.
We also need to know how to get them. Earlier (Posterior Analytics Book One, Chapter Three), Aristotle writes, ‘We … Holdthat not every form of knowledge is demonstrative, but that the knowledge of ultimate principles is indemonstrable. The necessityof this fact is obvious, for if one must needs know the antecedent principles and those on which the demonstration rests, and if inthis process we at last reach ultimates, these ultimates must necessarily be indemonstrable. Our view then is not only that knowledgeexists, but that there is something prior to science by means of which we acquire knowledge of these ultimates.’ So we cannotdemonstrate the first principles of a demonstration. How, then, do we arrive at them? This is the burden of Book Two, Chapter Nineteen below.