Proof within mathematics depends entirely on logic applied to previously agreed truths, whereas proof outside of mathematics often accepts data (evidence), likelihood, and precedents, as factors. Even so, the fact that the same word is chosen for both styles of thinking suggests that these two kinds of “proof” have something in common.Mathematics should not replace the kind of logic kids otherwise use, but should surely augment it, especially starting at a time when kids naturally “study” this kind of argumentation on their own! As students learn mathematical proof, they should consistently see how it relies on—and builds on and refines—their own good sense.Students don’t need to know the names of different kinds of proofs, or any such formalities. They do need to recognize the difference between a wild guess, a conjecture, and a proven assertion. And they do need to develop the inclination to wonder why things are as they are, to expect reasons, and—when possible—even provide a logical chain of reasons as the explanation. They also need to experience a variety of proof styles (expressed in developmentally appropriate ways). These might include any of the styles shown above: the constructive build-a-model approach with the Cuisenaire rods to investigate the skip-counting-by-three experiment; the raw logic (or exhaustive testing) that works on the Liar-Truthteller puzzles; and the healthy skepticism that leads students to find a counterexample in the perimeter and area problem.Habit of Mind 3: Distinguishing between Agreement and Logical NecessityLife outside mathematics involves rules, for example, the rule of not interrupting a person when that person is talking. This rule is purely a social convention, a “given.” It can even be broken without causing any trouble at all when friends are close and conversation is casual—that is, when all parties agree. The source of authority is just social agreement.Mathematics also involves rules. For example, in late elementary school, many curricula teach what is called “the order of operations.” In an expression like 3 + 4 5 + 1, one might imagine performing the arithmetic from left to right, adding 3 and 4, thenmultiplying by 5, and finally adding 1 to get 36 as a result. By agreement, however, we don’t do that: we perform multiplication and division before addition or subtraction. In this case, that means that the 4 5 is done first leaving us with 3 + 20 + 1, or 24, as aresult. Why this rule is chosen is pure convention, designed, like definitions, to serve only one master: usefulness. The source of authority is social agreement, for the sake of clear communication and useful results.Now consider another mathematical rule, the rule that two lengths may be added only if they are expressed in the same units. Three feet plus 24 inches is not 27 of anything: it is either 5 feet or 60 inches (or 1.666… yards, and so on). Likewise, three nickels and two dimes cannot be added until the units are the same: they can be jointly regarded as coins, in which case the sum is “5 coins” or they can jointly be regarded as cents, in which case the sum is “35 cents.” This rule is not based on social agreement; it is logically necessary in order for addition to make sense, and (therefore) it applies across the board to addition