How is 'is' known?

How does one know the term 'is' in a non-circular way? 

My first thoughts as I encounter the question. I think each persons beliefs interconnect that form a web. At the core of the web are beliefs that are the ultimate standard to interpret the web of beliefs. These core beliefs can be construed as either properly basic beliefs with defeating defeaters. Or can be understood as self-justifying as explanatorily ultimate.  Is it circular? Yes, but not in a vicious form. There are exclusions to the informal fallacy of begging the question. Walter Sinnott has a good article on this. I'd say I can define and explain the term 'is' but it will entail beliefs that lead to my core beliefs. Vincent Cheung would challenge my core beliefs with Scripturalism, infalliblism and internalism. I think I'd be immune to his criticisms, like AquaScum, if I take either epistemology (with reliabilism and externalism). I don't think an axiomatic system can evade admitting there is some circularity that is not vicious

It is quite simple to cite Webster's dictionary to define 'is'. But this will be an inadequate answer. Proof would be demanded to show the Webster dictionary 'is' a competent authority. Moreover, even if there 'is' sufficient proof the Webster dictionary 'is' a competent authority, the proper interpretation of the dictionary may further be doubted. A critic may always be critical, of anything, even of critical analysis itself. We are fallen creatures thus often fall into irrationality.  Hence, I think the question is an important one. In my judgment, God has created us with: properly functioning cognitive faculties, innate ideas, innate categories and innate grammar. Therefore, I follow in the tradition of St. Augustine. But I, as did Ronald Nash, depart from St. Augustine's insistence of denying any significant role of sense experience. I think we do, in fact, learn via reason (a priori) and sense experience (a posteriori). They are the secondary means for us to acquire true beliefs. But, I agree with Augustine, the primary means for us to acquire true beliefs is God. As we experience the world our minds filter and organize sense experience with our noetic structures (of innate ideas, categories and grammars) to acquire true beliefs. God sustains and secures our beliefs so as to match His thoughts with our thoughts (via the means mentioned). The inter-witness of the Holy Spirit testifies to the truth of Scripture. He is invincible thus defeating all defeaters. We acquire items of knowledge by a reliable belief-forming process. Furthermore, I take some beliefs to be properly basic (e.g. God's existence, an external world, other minds, sense perception, induction). Those beliefs are acquired by a reliable belief-forming process, independent of any arguments, and are rational to believe in absence of defeaters. 

The question can be answered directly. I learned the word 'is' by my parents and school teachers. I acquired the belief: 'is' refers to an English word that symbolizes the meaning 'to exist or be' or used as a designator. This belief was acquired via reason and sense experience, with or without my awareness, by a reliable belief-forming process and thus I'm rational to believe it in the absence of defeaters. But, perhaps, this is unsatisfying. Let's take a different approach. Logically prior to the question, is all forms of logical circularity fallacious? If you answer yes, then can you demonstrate this in a non-circular way? Counter arguments can be given that a valid distinction can be made between vicious and virtuous circularity (e.g. Armstrong, Frame). 

Remember the way to get out of the Clarkian system is to take up a different system. I don't think my view leaves us with the conclusion of the argument below, as Cheung would have us think: 

(1) If sense perception is necessary to interpret the Bible then sense perception is the definitive standard of truth not the Bible. 

(2) Sense perception is necessary to interpret the Bible (in any form of empiricism).

(3) Therefore, sense perception is the definitive standard of truth not the Bible. 

Premise (3) takes the primacy of sense perception and extra-Biblical ideas to be essential in constructing a system to best understand the Biblical teachings. 

The problem with this argument, besides the sloppy premises, is a conflating of ontic and epistemic priority. One can ontologically use sense perception to know a teaching of scripture without epistemically assuming sense perception is the definitive standard of truth. The point is one can build a system of thought placing the definitive truth in the scriptures and not the reverse (i.e.  Definitive truth placed in the means of knowing instead of the scriptures themselves as prerequisite for the possibility of knowledge).