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Visualizing Propositions

Suppose that PP and QQ are two sets of objects. Relative to these two sets, any object can be classified in exactly one of the following 4 ways:

  1. The object is neither in PP nor in QQ.
  2. The object is in PP but not in QQ.
  3. The object is in both PP and QQ.
  4. The object is in QQ but not in PP.

A Venn diagram is a picture that represents all the possible relationships between sets of objects. For example, the picture below is a Venn diagram for PP and QQ. The square represents all the possible objects, the circle labeled with PP are the objects in the set PP, and the circle labeled with QQ are the objects in the set QQ. The numbers refer to the above categories that describes the objects in the region.

venn1

There is an important connection between Venn diagrams and propositional logic. The key idea is that formulas of propositional logic describe regions of the Venn diagram: For all formulas XX and YY, the formula ¬X\neg X refers to the region outside the the region described by XX, the formula X∧YX\wedge Y refers to the region described by both XX and YY, and the formula X∨YX\vee Y refers to the regions described by either XX or YY. For example, in the image below, each of the 4 Venn diagrams is labeled with a formula describing the shaded region. Furthermore, each region can be associated with a row in a truth table for PP and QQ.

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Building on this idea, we can visualize other formulas of propositional logic constructed using the atomic propositions PP and QQ by shading different regions of the Venn diagram. Since there are 4 different regions of a Venn diagram for PP and QQ, there are 16 different ways to shade the Venn diagram. The key observation is that any formula of propositional logic that only contains the atomic propositions PP or QQ, describes one of the 16 possible shadings of the Venn diagram for PP and QQ. The image below contains all 16 different shadings, each labeled with a formula that describes the shading.

The Venn diagram at the bottom of the image has no regions shaded, the 4 Venn diagrams in the 1st row have exactly 1 region shaded, the 6 Venn diagrams in the 2nd row have exactly 2 regions shaded, the 4 Venn diagrams in the 3rd row have exactly 3 regions shaded, and the Venn diagram at the top of the image have all 4 regions shaded. There is a line drawn from a Venn diagram to a Venn diagram immediately above it when all regions shaded in the Venn diagram below are also shaded in the Venn diagram above.

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The above image is a very useful way to visualize important relationships between different formulas of propositional logic. There are three key observations:

  1. Any formula of propositional logic that only contains the atomic propositions PP or QQ is tautologically equivalent to one of the formulas labelling a shading of the Venn diagram in the following image.
  2. For all formulas XX and YY, if XX and YY only contain the atomic propositions PP and QQ, then X⊨YX\models Y whenever there is a path from a Venn diagram whose shading is described by XX to Venn diagram whose shading is described by YY.
  3. For all formulas XX, YY and ZZ, if XX, YY and ZZ only contain the atomic propositions PP and QQ, then XX is tautologically equivalent to Y∨ZY\vee Z whenever there is a line drawn from both the Venn diagram whose shading is described by YY and ZZ to Venn diagram whose shading is described by XX.

For example, the following facts:

  • P∧¬P⊨PP\wedge\neg P\models P,
  • P∧¬P⊨P∧¬QP\wedge\neg P\models P\wedge\neg Q,
  • P∧¬P⊨P∧QP\wedge\neg P\models P\wedge Q,
  • P∧Q⊨PP\wedge Q\models P,
  • P∧¬Q⊨PP\wedge\neg Q\models P, and
  • P≈(P∧Q)∨(P∧¬Q)P\approx (P\wedge Q)\vee (P\wedge \neg Q)

are illustrated by the highlighted lines in the following image:

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Tutorial#

Euler Diagrams#

It is important to note that in a Venn diagram, not all of the regions must contain objects. For example, suppose that we are considering sets consisting of the ten digits: 0,1,2,3,4,5,6,7,8,0, 1, 2, 3, 4, 5, 6, 7, 8, or 99. Suppose that PP is the first three digits. We express this by writing {0,1,2}\{0,1,2\}. The braces "}\}" and "{\{" are used to denote a set of objects (in this case, a set of digits). Suppose that QQ is {0,1,2,3,4,5}\{0,1,2,3,4,5\}. In the following Venn diagram for the sets PP and QQ, I list the set of elements in each region.

venn5

Note that the symbol "∅\varnothing" is used to describe the region consisting of objects that are in PP but not in QQ. This symbol means the empty set: That is, the set with no elements. Of course, there are no numbers in PP that are not in QQ since every number in PP is also in QQ. In this case, it is more convenient to draw the PP circle completely inside the QQ circle to indicate that everything in PP is also in QQ. When not all possible regions are depicted the diagrams are called Euler diagrams.

venn6

We can also use Euler diagrams to illustrate logical relationships between formulas of propositional logic. The idea is to treat a formula XX as the set of truth value functions that make the formula true. Then, X⊨YX\models Y means that every truth value function that is in the set of truth value functions making XX true is also in the set of truth value functions making YY true.

Example

Draw an Euler diagram of two formulas XX and YY when X⊨YX\models Y:

two propositions, valid

Example

Draw an Euler diagram of two formulas XX and YY that are mutually exclusive (i.e., there is no truth value assignment that make both formulas true):

two propositions, mutually exclusive

Note that there may be a truth value function that makes neither XX nor YY true.

Practice Questions#

  1. Suppose that there are three propositions AA, BB and CC. Draw a Venn diagram from these three prpositions and shade the region corresponding to (A∨B)∧C(A\vee B) \wedge C.
  1. Suppose that there are three propositions AA, BB and CC. Draw a Venn diagram from these three prpositions and shade the region corresponding to (A∧C)∨(B∧C)(A\wedge C) \vee (B\wedge C).
  1. Suppose that there are three propositions AA, BB and CC. Draw a Venn diagram from these three prpositions and shade the region corresponding to (A∧B)∨(A∧¬B)(A\wedge B) \vee (A\wedge \neg B).
  1. Suppose that there are three propositions AA, BB and CC. Draw a Venn diagram from these three prpositions and shade the region corresponding to A∧(A→C)A\wedge (A\rightarrow C).
  1. Suppose that there are three propositions AA, BB and CC. Draw a Venn diagram from these three prpositions and shade the region corresponding to B∧(A↔C)B\wedge (A\leftrightarrow C).
  1. Suppose that there are three propositions AA, BB and CC. Draw a Venn diagram from these three prpositions and shade the region corresponding to ¬(A→(B∨C))\neg (A\rightarrow (B\vee C)).
  1. Consider the following Venn diagram. What is a formula corresponding to the shaded region?

venn3, example6

  1. Consider the following Venn diagram. What is a formula corresponding to the shaded region?

venn3, example6

  1. Consider the following Venn diagram. What is a formula corresponding to the shaded region?

venn3, example6

  1. Draw a Euler diagram of two formulas XX and YY where X⊭YX\not\models Y and Y⊭XY\not\models X.
  1. Draw a Euler diagram of three formulas XX, YY, and ZZ such that X,Y⊭ZX,Y\not\models Z.
  1. Draw a Euler diagram of three formulas XX, YY, and ZZ such that X,Y⊨ZX,Y\models Z: