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What Is Infinity?One of the most difficult questions a curious student may ask a math teacher is whether Why this is a difficult question? It is difficult because it is usually asked by those who feel that infinity is a natural concept, like a number is, and everyone, especially a math teacher, should not have difficulty answering the question. But, for one, it is hard to insist that number is a natural concept. Indeed, while counting numbers have been realized by various cultures, zero and negative numbers were very long in coming, not to mention decimals and complex numbers. Secondly, even for those numbers that are perceived natural, the concept of division is not fundamental and has to be defined in the course of a study. Addition for natural numbers is an outgrowth of counting and may be easy to define (or accept) for the numbers 1, 2, 3, ... But all the rest (meaning other numbers and operations, like subtraction, multiplication, division) require a definition. So, in the absence of any preliminary understanding or common knowledge, the best (but seldom expected or acceptable) answer is, What do you mean by ∞ or, for that matter, by dividing by 0? Thirdly, even assuming there is a useful definition of infinity that deserves a recognition and a symbol of its own, it is not yet obvious that for this infinity it is possible in a reasonable manner to define arithmetic operations with more common numbers. Finally, there are many infinities in mathematics; and this is true in more than one sense. There are many infinities and, say, ∞ is not a common notation used to denote each of them. Besides ∞, other symbols, for example, It is prudent then to deal with various infinities one at a time. We shall look at several
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and ω, are in circulation that denote infinities very much different from the one (or ones?) that ∞ usually stands for. Various infinities are defined differently and are subject to different operations and different laws. For example, while 
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