Aristotle, Posterior Analytics (aka APo), 72b5-27:

• Now some think that because one must understand the primitives there is no understanding at all; others that there is, but that there are demonstrations of everything. Neither of these views is either true or necessary.
• For the one party, supposing that one cannot understand in another way, claims that we are led back ad infinitum on the grounds that we would not understand what is posterior because of what is prior if there are no primitives; and they argue correctly, for it is impossible to go through infinitely many things. And if it comes to a stop and there are principles, they say that these are unknowable since there is no demonstration of them, which alone they say is understanding; but if one cannot know the primitives, neither can what depends on them be understood simpliciter or properly, but only on the supposition that they are the case.
• The other party agrees about understanding; for it, they say, occurs only through demonstration. But they argue that nothing prevents there being demonstration of everything; for it is possible for the demonstration to come about in a circle and reciprocally.
• But we say that neither is all understanding demonstrative, but in the case of the immediates it is non-demonstrable--and that this is necessary is evident; for if it is necessary to understand the things which are prior and on which the demonstration depends, and it comes to a stop at some time, it is necessary for these immediates to be non-demonstrable. So as to that we argue thus; and we also say that there is not only understanding but also some principle of understandng by which we become familiar with the definitions.
• And that it is impossible to demonstrate simpliciter in a circle is clear, if demonstration must depend on what is prior and more familiar; for it is impossible for the same things at the same time to be prior and posterior to the same things...

1. Some think: 72b5
   
1. One must understand primitives in order to understand anything.72b5
2. One cannot understand primitives (unstated premise that makes 3 valid)
3. Therefore, by 1.1 and 1.2,  there is no understanding. 72b5-6
2. Others think: 72b6
1. There are proofs of everything. 72b6
2. Proofs involve/lead to/show understanding. (unstated)
3. Therefore, by 1 and 2, there is understanding. 72b6
3. Some:
1. There is no other way of understanding other than demonstration. 72b8
2. Demonstrating that p involves q and ...: demonstrating q involves r and ...: demonstrating r involves s and ...: ... (implied by 72b8-10)
3. A primitive is something has no or requires no demonstration. (implied 72b5ff.)
4. By 3.2, if there are no primitives, demonstration will never stop. 72b8-10
5. By 3.3., if there are primitives, demonstration will stop. 72b11
6. By 3 and 1, there can be no understanding of primitives. 72b11-12
7. If there is no understanding of primitives (3.6), then there is no full unqualified (simpliciter) understanding of anything that is demonstrated by using primitives. 72b13-14
4. Others:
1. 3.1 is right: there is no understanding without demonstration. 72b15
2. 2.1 is right, because 'nothing prevents it.' 72b16-17
3. Therefore, understanding is achieved by circular/reciprocal reasoning. 72b18-19
   
5. We say about 'some':
   
1. Understanding a conclusion of a demonstration requires understanding the reasons for that conclusion, and understanding those reasons requires understanding the reasons for those reasons, etc. 72b21
2. Understanding via demonstration does come to a stop. 72b22
3.  Therefore by 2 and 3, there must be some other way of understanding than by demonstration. implied in 72b23
4. Not all understanding is demonstrative. 72b19
5. Immediates are not demonstrable, but are understandable by definition. 72b10
1. immediates are things that are not understood via some middle thing, via reasons for them: they are understood immediately. implied in 72b20-24
   
6. Therefore, by 5.5, which refutes 2.1 'some' do not have a true and necessary view. 72b7
6. We say about 'others':
1. Being prior means coming before in some fashion. implied in passage
   
2. Being posterior means coming after in some fashion. implied in passage
3. It is impossible to be both before and after the same thing in the same fashion. 72b26
4. Demonstration necessarily requires using a prior thing to prove a posterior thing (that is definitional of demonstration). 72b25-27
   
5. Therefore by 3 and 4, the circular or reciprocal reasoning claimed in 4.3 is impossible.