Absolutely elementary mathematics originates from counting by one. In math we want certainty. But certainty does not come cheap. A relatively obvious statement may involve long proof with lots of details.
Knowing that Russell wrote Introduction to Mathematical Philosophy while in prison (like Nehru's Glimpses), creates a renewed urge to read the work. The work (among other things) tries to explain natural numbers without invoking the concept of number. What makes three sheep, "three"? It's its similarity with other sets of three things. Each thing in a pair of three-sets can be matched elementwise, exactly. Four-sets can also have this matching, then how do we differentiate between three and four? Three-set is bigger than two-set and smaller than four-set. Eventually we end up recognizing a one-set or even an empty set? In any case, the numbers are then properties of not objects but sets of objects.
Membership, are you in or out, is the fundamental relationship in set theory.
Say 0 represents the empty set. Then we can build a sequence of sets like {0}, {0, {0}}, {0, {0}, {0, {0}}}, ... which corresponds to the natural numbers 1, 2, 3, ...; for counting we can try matching , say the apples, to the elements of each set in the sequence and whichever matches exactly the number of apples then is the same as the number of elements in that matching set.
Validity of a logical argument depends on form not content. First check premises and conclusion are in right form. Then comes checking the truth of the premises which is outside of logic.
Author seems to hold the opinion that proof is the most distinguishing characteristic of math because no other field has "proof". Given that a math proof, in the end, is a sociological construct (seems very successful though), I would say "proof" is what a field has established as a process to agree if a statement is true or false. In that sense, science has its own sort of proof.
The five Peano axioms are like five Euclidean ones, but for natural numbers. The central focus of the axioms is the idea of "succession". Dedekind took counting as fundamental, and from that numbers come forth.
To define x+y in terms of "succession" we need three axioms: (1) x+0=x (2) x+z=x+(y+1) (3) x+z=(x+y)+1; by the third we have actually defined x+z as the successor of x+y. As an example, 4+3 = 4 + (2+1) = (4+2) + 1. If we continue: 4+2 = (4+1) + 1, 4+1 = (4+0) + 1; so 4+3 = (4+0) + 1 + 1 + 1. Applying the same for 4, we have 4+3 = 0 + 1 + 1 + 1 + 1 + 1 + 1 + 1 or with S as successor 4+3 = SSSSSSS(0). This last one is what kids do, they collect the 4 and 3 objects in a pile and count.
To define multiplication we assume addition has already been defined and we have three more axioms: (1) x * 0 = 0 (2) x = y+1 (3) x * y = x * (y+1) = x*y + x. These operations on natural numbers are defined recursively. Kleene's recursive theorems are important in this respect.
Law of induction can be proved from the assumption (well ordering principle) that every non-empty set of natural numbers contain a smallest element. Well ordering principle can be used to demonstrate that there isn't a natural number between 0 and 1.
Noether established the link between symmetry and conservation laws. She also brought rings to the front. Using the axioms of rings we can demonstrate that (-x)(-y) = xy. Polynomials form ring. Fractions form a field.