So, fractions must follow the rule: halves and thirds cannot be added until they are converted to the same units. One such common unit is sixths: three sixths and two sixths are five sixths. The same goes for the standard algorithm for adding two multi-digit numbers. The reason to line up the columns is not because “that’s the way it’s done” but because that helps make sure one only adds things that are expressed in thesame units. The reason this “sum of 120 and 43” is wrong is that it tries to oops! add a 2 and a 3 (and a 1 and 4) that represent different things. The 2 couldbe tens, or dimes, and the 3 is ones, or pennies. The source of authority for all these addition rules is logical necessity. You can’t add apples and oranges!Why is it so important to recognize the source of authority—convention or logic—for each mathematical idea? Because one goal of school mathematics is to support the development of children’s reasoning. If all rules are arbitrary, or if no distinction is made, rules become divorced from, or even the enemy of, common sense. How often do we see people use (or cave in to) data and graphs, even when the “math” does not really support the argument!The fact that three-sided polygons are called triangles (among English speaking people) is also social agreement. So is notation, like the convention that the vertices of a triangle are labeled with capital letters, like A , B , and C , or that a side of ABC is written as AB , not ab or AB . The source of authority for 360° around a point (90° in a right angle) is that “that’s the way we define the words degree and angle and right angle,” and so on—again social agreement.But the fact that the three angles within a triangle have the same total measure as two right angles (what we call 180°) is not a matter of social agreement, or even a “natural law” discoverable by experimentation. It is logically necessary: one can show (prove) that the angles must add up that way. Most often, you are shown this necessity by a proof in a geometry text, but if you know the conventions (and have a bit of experience doing the math), you can reason out this result yourself.Conventions, like people’s names and addresses and phone numbers, may have patterns and reasons and history, but not logical necessity. Mathematics provides a beautiful counterpoint, an opportunity to see that some truths—such as the fact that the sum of two odd numbers must be even—are ones that students can reason out for themselves. Mathematics gives them a chance to practice that kind of reasoning. And only in mathematics is this appeal to logic-alone possible.Students commonly ask, in any subject: “Did I do it right?” “Is my answer correct?” Mathematics often lets students answer such questions themselves, helping them build a certain independence of learning and a confidence that can have benefits far beyond the math lesson. The student can say, “I can figure this out by myself, I don’t need books or authority figures. I can be the authority.” Seeing when this is true also help students learn to recognize when it is not true. Convention, cultural legacy, data from history or the physical world, and so on, are different, and require the authority of reliable outside sources (books, people, experiments).