Reasoning and Proof
Explanations and development of reasoning, proof, & logic


Proof is the essence of mathematics - it is what makes mathematics valid. The making of conjectures about the use of mathematics and the justification of conjectures as solutions. Anything less can not present the true nature of mathematics. Classrooms must empower students to make conjectures, create solutions, and justify the methods for attaining the results; instead of focusing on justifying their use of steps from teacher taught procedures.

While mathematical proof validates some kinds of conjectures as solutions, not all questions, problems, or investigations require a formal logical proof. Other ways of making decisions or justifying them can also be useful and appropriate in different situations.

See also:

Proof as common sense

Proof - An explanation accepted by a community at a given time. The rigor of the proof is dependent on the members of the community. Communities can range from a kindergarten class as a community where proof is relative to what a particular child thinks; to a community of mathematicians, who require a formal logical proof.

Mathematical related definition

Proof is a sound, convincing, and unquestionable explanation, for anyone who understands the concepts, and for which no counterexample can be given. It can be provided for explanation, communication, verification, systematization, and discovery.

Types of proof

Visual representation as proof

Visual comparison is the first kind of proof offered to and demonstrated by children.

  • Look! Is it a dog?
  • Is it a circle?
  • Is it a triangle?
  • It's a square see.

Young children look at an object and compare it to their mental visual representations. If it looks like one of their previous minds-eye views, that is proof enough. They do not use properties as justification. It is a holistic visual comparison.


Comparison of properties as proof

The identification and use of properties appear as children begin to take closer looks at objects, images, or visual representations. They begin to compare properties or characteristics of the objects along with the images. For example, different and similar properties of dogs, cats, cow, pigs, squares, triangles, and other objects are added to their mental images. Images of dogs with four legs. Dogs with fur. That bark. The number of sides on squares, triangles, and circles. How the sides are curved or straight.

The mapping of properties from a person's mental representation of an object to a questionable object is an example of comparison being used as evidence to create a proof of equivalence or not. This use of matching properties or mapping properties from one to another to use as evidence to build a proof is rarely used by children. Children and even adults persist on using observations. If it looks like a cat, then it is a cat. for example teachers' attempts to teach children to use scientific classification as a way to identify living organisms often results with students' persistent claims that dolphins and whales are fish, snakes don't have bones, and humans aren't animals.

However, with continued experiences of having students identify properties, classify by properties, and organizing solutions based on properties, students will increasing use this as a strategy of proof. In geometry, for example, students can be asked to identify properties and use those properties to identify and organize figures and shapes by those properties. With these experiences students will learn that some properties are necessary for an object to belong to a group and others are not. They will learn that there are properties that are independent of the conditions they are exploring. They will learn that there must be a sufficient number of properties needed to describe or identify a particular object or groups of objects. And they will learn that if two objects have the same necessary and sufficient properties, then they are equivalent.

Stated another way - To prove one object as a member of a class or congruent with another object, then all of the necessary and sufficient properties of the object, class, or group must be identified for the questionable object.

Lets explore these ideas for the class of objects known as triangles. What are the necessary conditions for a triangle?

Necessary properties/conditions/characteristics of a triangle

  • Plane figure
  • Closed figure
  • Three sides
  • All sides are straight lines

Not Necessary properties/conditions/characteristics of a triangle

  • Three dimensions figure
  • Open figure
  • Four sides
  • One side or more has a curve

Which of the following necessary conditions must be included to include all the sufficient conditions needed to describe a triangle or for a figure to have to be a triangle?

  • A plane figure - Not sufficient, because there are plane figures that are not triangles
  • A closed figure - Not sufficient, because there are closed figures that are not triangles
  • Three sides - Not sufficient, because there are figures with three sides that are not triangles. In fact all polygons have three sides and more
  • All sides are straight lines - Not sufficient, because there are other shapes with all sides being straight lines.


Does this list include sufficient conditions or all the minimum number of necessary conditions to represent a triangle?

The list is sufficient And all are necessary for a plane figure to be a triangle or for the list to be used to define a triangle.

This comparison can be used to categorize or group objects by their common properties.

Or when considering if two things are equivalent or congruent. Triangles with identical necessary and sufficient properties are congruent.

Example of equivalent descriptions of numbers.

Two sets of properties that represent a whole number.

  1. Set one - It is greater than one, it is less than three, it is a whole number. [2]
  2. Set two - It is the smallest positive whole number that is also an even number. [2]

To prove these are equivalent conditions we must show they are both necessary and sufficient. They are as they lead to the same conclusions. This would make it possible to write: the whole number greater than one and less than three is equal to the smallest positive even whole number. Further it would also be valid to set both equal to 2.

Take your time and make this information YOURS.

If you do, then you can use it to create questions to probe your students' depth of understanding. By having them consider what is necessary? What is sufficient? And what more needs to be added to make something sufficient that is not? What statements or groups of statements are equal or equivalent? What does it look and sound like if we put them together as equals?


Proof by disproving with a counterexample

We make a conjecture and find an example that doesn't fit the conjecture, then can claim we have disproved the conjecture by discovering a counterexample.

Example 1

  • Conjecture - The squares of all numbers are odd.
  • Example - Three squared is nine.
  • Counterexample - Two squared is four.

Example 2

  • Conjecture - The squares of all numbers are even.
  • Example - Two squared is four.
  • Counterexample - Three squared is nine.


Deductive proof

Deductive proof - uses a general definition to include a specific instance or a premise to reach a conclusion. Is generalizable to all instances with the same characteristics that satisfies the givens or represents the given class, definition, or law.

Start with a set of assumptions and use them to derive a valid conclusion.


  • It is a whole number
  • It is a perfect square
  • It is between 40 and 100
  • It has a factor of three

What is it?

Specific instances

  • Whole numbers are 1, 2, 3, 4, ..
  • Perfect squares are 2, 4, 9, 16, 25, 36, 49, 64, 81, 100 ...
  • Whole number perfect squares, between 40 and 100 are 49, 64, 81

Conclusion - The only one number of 49, 64, and 81 to have a factor of three is 81.

Logical Reasoning is an example of deductive proof

Four Logical General statements with examples
  1. Proposition: If you do X, then you may do Y.
  2. Inverse: If you do NOT do X, then you may NOT do Y.
  3. Converse: If you do Y then you do X.
  4. Contrapositive: If you do NOT do Y then you will NOT do X.
Specific examples
  1. Proposition: If there is rain, then there are clouds.
  2. Inverse: If there isn't rain, then there aren't clouds.
  3. Converse: If there are clouds, then there is rain.
  4. Contrapositive: If there aren't clouds, then there isn't rain.
Statements equivalent to the proposition:

Proposition: If there is rain, then there are clouds.

  • Rain implies there are clouds.
  • If there is rain, then there are clouds.
  • There is rain only if there are clouds.
  • There are clouds if rain.
  • If there are no clouds, then there is no rain.
  • Rain is sufficient for clouds.
  • Clouds are necessary for rain.
Statements not equivalent to the proposition:

Proposition: If there is rain, then there are clouds.

  • If there are clouds, then there is rain.
  • If there isn't rain, then there aren't clouds.
  • There are clouds only if there is rain.
Statements coded:
  1. Proposition: If X, then Y.
  2. Inverse: If ~X, then ~Y.
  3. Converse: If Y, then X.
  4. Contrapositive: If ~Y, then ~X.

Equivalent statements:

X -> Y
if X, then Y
X only if Y
Y if X
~Y -> ~X
X is sufficient for Y
Y is necessary for X

Examples - Label each of the following statements from the proposition and tell which is true if the proposition is true.

Proposition: If John has work to do, then he is in his seat.

If John doesn't have work, then he isn't in his seat.

If John is in his seat, then he has work to do.

If John isn't in his seat, then he doesn't have work to do.

Label each of the following statements from the proposition and tell which is true if the proposition is true.

Proposition: If Mary doesn't do her work, then she will not go to recess.

If Mary does her work, then she will go to recess.

If Mary will not go to recess, then she did not do her work.

If Mary goes to recess, then she did her work.

Write four of your own.


1. Marty asked her teacher if she should do a report. The teacher said, "Marty, you can earn an A only if you do the report." Marty did the report, but she did not get an A. Did the teacher lie?

2. A teacher told Jim that if he didn't do his work he wouldn't go outside to recess. Jim didn't do his work and the teacher let him go to recess. Did the teacher lie?

More examples

Assume that the first statement is true, tell if the following statements are true, false, or questionable.

1. If you wear nice clothes to school, then your teachers will be impressed.

2. If your teachers will be impressed, then you wear nice clothes to school.

3. If your teachers won't be impressed, then you don't wear nice clothes to school.

4. If you don't wear nice clothes to school, then your teachers won't be impressed.

For the four statements above this time assume the first statement is false, now tell if the same three statements are true, false, or questionable.

Common errors in logical reasoning:

Circular reasoning:

James: "Grass is green."

Lynn: "Why do you think so?"

James: "Because it is green."

Proof by selected instances:

John has never seen his parents fight. He thinks that all married couples never fight.

Proof by failure to find a counter example:

Mary has never seen a purple elephant, therefore they do not exist.

Proof by counter example:

You are never on time. What do you mean? I was yesterday.

Avoiding the question:

Is it bad doc? It's too early to say.

Special pleading:

Dad: "Slow down your going 60 mph and that's over the speed limit."

Son: "You drive 65 mph".

Dad: "That's different. I know what I'm doing."

Faking a connection:

A dog is an animal. A cat is an animal. Therefore a dog is a cat.

Don't forget substituting the converse or inverse for the proposition.


Tell if the following are logical or not and what kind of faulty reasoning is being used, if they are faulty.

1. It doesn't make any difference to me whether you believe me or not, because I don't care one way or another whether or not you believe me.

2. Development of solid foundations of reasoning is essential to the proper education of a person. Therefore, if someone is to be educated properly, then she must at least acquire a thorough knowledge of the basics of reasoning.

3. "Dad, can I use the car tonight?"
"What do you think?"
"Nope. You told me not till next week."

4. I know that I can not learn a foreign language, because when I took Spanish I received an F.

5. My mother can cook anything. She is always trying new recipes and they always turn out great.

6. Daughter: "Why won't you let me go downtown? You said you did when you were my age.
Mother: "That's different. The times have changed, the size of the town, and the ways of the people."

7. John must be a very good person, because I have never heard anything bad about him.

8. "Sara, will you go out with me tonight?"
"Get lost."

9. All teachers are important people. You are a teacher. Therefore, you are an important person.

10. "Mom, Why is the sky blue?"
"Ask your dad."

11. "Mom, why is the sky blue?"
"because red wouldn't look so good."

12. "Mom why is the sky blue?"
"I don't know."

13. All traditional teachers favor corporal punishment. George favors corporal punishment. George is a traditional teacher.

14. You said you would let us go to recess only if we did all our work, and we did all our work, so now you have to let us go to recess.

15. Teacher: "I can not let you use the phone, because the rule says no one uses the phone during class time."
Student: "Please just for me, I won't tell anyone and I have to make this call before mom leaves for work."

16. All teachers like children. You like children. So you are a teacher.


Inductive reasoning

Inductive reasoning - is using the specific to create the general or conclusion. Rolling two die hundreds of times to find the probability of the different sums. Inductive reasoning is the step child of deductive reasoning since it is based on experiment and variables. For example - statistical reasoning is not accepted in a courtroom.

Reasoning and proof - if you include ideas of sufficient, necessary, equivalent, and independent conditions, you have the tools for reasoning and proof.

Independent conditions are properties or conditions that vary independently of the other.

Example size and shape. It is possible for the shape of a square to have an infinite variety of sizes and still have the shape of a square. Similarly the properties necessary to make a shape a square do not include size.


Proof by analogy or metaphor

Proof by analogy or metaphor - mathematicians might distinguish analogies as a directly related comparison of two like arguments and metaphor as indirectly related.


  • Feathers are to birds as scales are to fish
  • Feathers are like scales (body coverings) and birds are like fish (animals).
  • Feathers : bird :: scales : fish :: hair : mammals
  • Den:fox :: hive:bees
  • Bird :: nest:: burrow:rabbit

The relationships are obviously clearly definable relations of the respective parts or artifacts to animals as well as parts or artifacts to parts or artifacts and animals to animals.

Metaphors are not as tight - At night all cats are black. Can not substitute - like the previous analogies. There is a violation of syntactic structure. Night relates to black as light or dark and black relates to cats as to color - white, brown, yellow, black; while the figurative meaning of the metaphor is something unrelated.


Proof of the Commutative Property

To show students a specific example, can be a starting point of a proof. Specially for very young children. Like 3 + 5 = 5 + 3; can be demonstrated by gluing three beans to card stock and five beans to card stalk, then showing that either order of the cards has a cardinality of eight beans. However, if we stop here we must recognize it is not a proof. In fact it is recognized as not being a proof - by attempting to prove a specific example or a selective instance as equivalent for all possible examples.

To make it a proof the idea must be generalized for all possible cases or the infinite number of cases that might exist. This can be suggested by asking what happens if the number of beans are increased. Again, young children usually reach a limit because of their limited understanding of the number system. However, as students understanding of number systems increase toward a conceptualization of infinity, the generalization can be pushed toward infinite possibilities and the equivalence for all of them. For children to be able to do this they will need to have achieved all the conditions for conservation. When you believe this is so, the next example can be presented.

Prove that when you add any two numbers the sum will be the same no matter the order of the numbers. While all the students may be convinced of this it is necessary to push them to explain why it would work for addition of all numbers (infinity again) for the case of whole numbers. Have them create an explanation that they could give to another person (art, music, PE. teacher or principal).

To move to infinity students should be able to show with manipulatives and a drawing that represents additional or less numbers. You can question by asking for repeated larger or smaller numbers.


More examples with proof and more examples to prove

Geometric construct of a Proof. imageProve that a construction of a triangle will yield an equilateral triangle. Figure at right shows how a compass can be used to demonstrate how three equal lines can be drawn to create a triangle with all three sides congruent.

Proof of when an Even number is added with another even number it will make an even number. See three different levels: concrete, semi-concrete, and formal proofs .

Proof of a specific and translate to general

Students found the mass of a dry sponge (98 g), then soaked it in water and found its mass (145g). When they were asked to find the mass of the water they wanted to subtract. The teacher asked them to prove it or convince her that it was an accurate solution.

The wet sponge's mass was 145 grams - the dry sponge's mass 98 grams. Students decide to use nice numbers and adjust to subtract. So they subtracted 100g. 145 - 100 = 45. Then they were not sure if they should subtract two more or add two more.

To represent the problem they drew a rectangular shape (sponge) with an enclosed blob to represent the water. They labeled the area outside the blob 98g for sponge and imagelabeled the blob - water with a question mark. Then they wrote 145g beside the rectangular shape and said that represented the sponge and the water.

They proceeded to explain that taking 100g away was 2g too much so they would need to add two back.

To get to the generalization the teacher asked this follow up question: Would this always be true for subtraction?

What would the statement be? Or rule?

Like - If you take away smaller you end up bigger so you need to ... Or If you take away bigger you end up smaller so you need to ...

With the hopes to get to the idea of: How to argue for an infinite claim.

Think that problem is too complicated to start?

What if we use the same numbers only change the scenario?

Max has 145 collector cards and gives 98 to Chris. How many collector cards does Max have left?

This can also be solved with subtraction and the numbers are the same as the sponge problem. However, the representation of the problem isn't as complicated because of the type of subtraction problem it is.

Why is there such a difference? The sponge problem is stated as an addition problem with the start and finish numbers know, but not the change. This makes it more complicated than this second which is a start total known and the amount to remove or separate known with the unknown as what is left.

See samples of different addition and subtraction problems

The following are examples of specific instances that can use proof by analogy if the analogy is stated as to be infinite.

Prove why you can exchange numbers of equal value with equations of equal value.

Prove the validity of this equation:

Prove that two unknowns x and y when added together and squared are represented accurately with the following:

Can prove the validity of the distributive property by analogy?

x2 - 1 = (x +1 )(x - 1)

Teaching suggestions

Arguments from children's representations (like the 3 + 5 = 5 + 3 ) and from other student stories where numbers are involved can be an effective way to move students toward a proof as a more general idea. The key is for the teacher to facilitate the inclusion of questions such as:

The purpose is to push thinking to generalize the explanation to include all possible examples that are true for all cases, or move to infinity.


A = length * width

Does multiplying length and width determine area for every rectangle and square? Why?

We can use graph paper or tiles to represent a rectangular area. By modeling how every time a row of square tiles is add it is represented by multiplying the number of column by an increased row number of one.

Elementary students will need to act this out multiple times. Talking about how the rows are increasing and the columns are increasing and how that can be represented in the product of the length and width. Then ultimately they should recognize that it doesn't matter what the number of rows or columns are. Therefore, the expression L * W will determine the area. To be able to do so students need to be familiar with all the necessary conditions or characteristics for conservation. Specifically in the case of conservation of area.

Proof by Exhaustion

Identify all examples and justify their inclusion as well as reasons for the elimination and identification of all possibilities.


What are all of the possible four cube high towers that can be made with red and yellow cubes?

Proof by exhaustion using systematic process of recursion.

Students must take time to build meaning, which must be from concrete experiences. Time to notice patterns between the physical entities and retain these ideas in short term memory long enough to act on them to create relationships and communicate these relationships.

It is the communication element that pushes us to identify the limits, systematically explain all possibilities even if they extend to infinity.

There are several ways to systematically try to insure the discovery of all possibilities of the cube towers. Some examples follow.

First towers can be made to show the number of red and yellow cubes that could be to make a four cube tall tower. Below illustrates that if four cubes are used for each tower it would be possible to build a tower with 4 red, 3 red and 1 yellow, 2 red and 2 yellow, 1 red and 3 yellow or 4 yellow. There is no other combination of reds and yellows for a 4 cube tower.

imageHowever, that doesn't provide for the different placements of the two colors in different combinations. For example: There is only one way to arrange a tower with all red. Like wise there is only one to arrange a tower with four yellow. However, there are several ways to arrange three red cubes and one yellow cubes. Like wise for two red and two yellow and three yellow and one red.

These can be found by rearranging the cubes for each of the above combinations of colors and summing the totals 1 + __ + __ + __ + 1 = __ .

Another way is to use a tree. Start with R and Y and put a R Y pair below it and continue till there is a row for each of the four rows in the towers. Then find all the different paths through the maze of connections and label each.



Another example by exhaustion or discovery is all the examples to find,
How many squares in a 5 X 5 square?

Systematic solution could find the number of 5 x 5 grid

Then total the five groups.

See example of card trick explanation using elimination and identification.

Prove the handshake problem

How many handshakes will there be for everyone to shake hands with everyone else in your classroom.

Systematically - if there are five people, the first person shakes hands with the four other people in the classroom. Since the first person has shaken everyones hand, the first person can relax and sit down. The second person has shaken the first person's hand, so the second person begins by shaking the third person's hand. When the second person has shaken everyones hands the person may relax and sit down. The third person now has shaken hands with person one and two, so has to shake the fourth and fifth persons' hands. Then the fourth person only has to shake the fifth person's hand and when it becomes the fifth persons turn, by golly the fifth person has shaken all the hands and is done. To summarize.
Person five - 4, person four - 3, person three - 2, person four - 1, person five - done - o.
Add them and get: 4 + 3 + 2 + 1 = __ .

Misconceptions and fallacies

Ecological fallacy

Ecological fallacy is thinking relationships observed for groups necessarily hold for individuals.


Exception fallacy

Exception fallacy is sort of the reverse of the ecological fallacy. It is when a conclusion about a group is made on the basis of a specific case. This fallacious reasoning is often at the core of sexism and racism. The stereotypical response when seeing a woman make a driving error of: "women are bad drivers." Is wrong. Fallacy...

Scoring Guide for proof

Proof Production Level 0

Students at this level appear unaware of the need to provide a mathematical justification to demonstrate the truth of a proposition or statement. For example, a student might accept a proposition as true because a teacher, parent, or text "says" it's true (cf. Harel & Sowder, 1998); in this case, the justification is "non-mathematical." In other cases, a student might simply state a proposition is true without any reference to why the proposition is true (e.g., "the sum of two even numbers is even because that is just the way it is," "yes the numbers will be equal because they will always be equal").

Proof Production Level 1

Students at this level appear to be aware of the need to provide a mathematical justification, but their justifications are not general; in the majority of cases, students' justifications are empirically based. Among the empirically based justifications, we recognize distinctions (at sub-levels) among students who consider checking a few cases, students who consider systematically checking a few cases (e.g., even and odd numbers), students who consider checking extreme cases or "random" cases, and students who consider the use of a generic example (proof for a class of objects) (cf. Balacheff, 1987).

Proof Production Level 2

Students at this level appear to be aware of the need for a general argument, and often attempt to produce such arguments themselves; the arguments, however, fall short of being acceptable proofs. "Falling short" may happen in one of two ways: (1) Students express recognition of the need to provide a general argument and attempt to produce such an argument, however, the argument provided is not a viable argument (i.e., the argument is either incorrect mathematically or it would not lead to an acceptable proof). (2) Students express recognition of the need to provide a general argument and attempt to produce such an argument, however, the argument is incomplete (if completed, the argument would be an acceptable proof). In both situations, the point is that students are attempting to treat the general case. In addition. Level 2 justifications also include responses from those students who demonstrate an awareness that empirical evidence does not suffice as proof—by either expressing recognition of the need to deal with all cases or expressing recognition of the limitation of examples as proof—but who are unable to produce (or attempt) a general argument.

Proof Production Level 3

Students at this level appear to be aware of the need for a general argument, and are able to successfully produce such arguments themselves. We consider the arguments students produce at this level to be acceptable proofs; that is, their arguments demonstrate that a proposition or statement is true in all cases. Arguments categorized at this level typically involve a reference to any assumptions or givens, a chain of deductions used to build the argument, and finally an explicit concluding statement. Although the arguments students produce may lack the rigor or formality typically associated with a proof, their arguments, nonetheless, do prove the general case.

See the original article for explanations and examples of the levels.
From Middle School Students' Production of Mathematical Justifications. Eric J. Knuth, Jeffrey M. Choppin, and Kristen N. Bieda. In Teaching and Learning Proof Across the Grades. A K-16 Perspective (2009).



Research suggests current practice is insufficient and only change can improve students abilities to reason and use proofs. Goswami, and Hatano and Sakakibara (2004, ISBN 0-8058-4945-9.) observed children in natural settings using everyday language while they were attempting to use the language of mathematics while reasoning mathematical. That language was mostly absent of spontaneous analogies. The authors attribute these findings to limitations in a child's mathematical knowledge as well as to the norms and practices in the students' instructional setting.

A mathematical community or culture must include many opportunities with experiences that emphasize problem solving, representation, reasoning, and communication, where students make conjectures and develop skill in using these tools of reasoning and proof through systematic representation to justify their ideas.


Dr. Robert Sweetland's notes