Chapter 9: Tree Diagrams for Arguments

How to Build an Argument Tree (pp. 71-75)

• Reconstruct the argument, numbering the premises and conclusions in a passage.

(1) Premise

(2) Premise

(3) Premise (however many premises)

(4) Conclusion• Place unstated premises or conclusions in

square brackets ([ ]).

How to Build an Argument Tree (pp. 71-75)

• Numbers representing premises are placed above numbers representing conclusions. – If two or more premises jointly support a

conclusion, use a diagram that looks like a goal post: └┬┘, └┼┘, └┴┬┴┘, and so forth.

– If two or more premises individually support a conclusion, draw a line from each premise to the conclusion.

How to Build an Argument Tree (pp. 71-75)

• All deductive arguments and some inductive arguments have conclusions jointly supported by premises – In analogies the premises jointly support the

conclusion.– In arguments to the best explanation the

premises jointly support the conclusion.– In inductive generalizations the premises individually support the conclusion.

Examples of Argument Trees

• An inductive generalization: (1) Joanne is a ΔΔΔ and attended the meeting.

(2) Luisa is a ___ and attended the meeting.(3) Bianca is a ΔΔΔ and attended the meeting.(4) Nadine is a ΔΔΔ and attended the meeting.So, (5) all ΔΔΔs attended the meeting.

(1) (2) (3) (4)

(5)

Examples of Argument Trees

• An analogy:(1) Jose is like Luis insofar as both were born in New York City, both enjoy classical guitar, and both enjoy the occasional evening at Lincoln Center.(2) Jose likes to spend an occasional evening at Carnegie Hall.Therefore, Luis likes to spend an occasional evening at Carnegie Hall.

(1) (2)

(3)

Examples of Argument Trees

• Argument to the best explanation (schema):(1) Both hypotheses A and B explain phenomenon P.

(2) Hypothesis B is a better explanation than hypothesis A (on specifiable grounds, see Chapter 27).

So, (3) hypothesis B is more probably correct (true).

(1) (2)

(3)

Examples of Argument Trees

• A deductive argument: (1) If argument trees are fun, then you’re strangely amused.

(2) You are not strangely amused.

Ergo, (3) argument trees are not fun.

(1) (2)

(3)

Longer Passages (pp. 76-82)

• Longer passages contain several arguments in which the conclusion of one is a premise of another.

• You proceed in the same way, indicating that the conclusion of one argument is a premise for the next.

• There can be any combination of straight branches and split branches.

Example of a Long Passage

(1) Amy went to the dance. (2) If Amy went to the dance, then all members of ΚΚΨ went to the dance. (3) Gertrude is a member of ΚΚΨ, and Gertrude went to the dance. (4) Carissa is a member of ΚΚΨ, and Carissa went to the dance. (5) Elena is a member of ΚΚΨ, and Elena went to the dance. (6) So, all members of ΚΚΨ went to the dance. (7) If all members of ΚΚΨ went to the dance, then all members of ΠΡΣ went to the dance. (8) So, all members of ΠΡΣ went to the dance. (9) Either not all members of ΠΡΣ went to the dance (some members of ΠΡΣ did not go to the dance) or some members of ΜΜΠ boycotted the dance. (10) So, some members of ΜΜΠ boycotted the dance.

Example of a Long Passage(1) Amy went to the dance. (2) If Amy went to the dance, then all members of ΚΚΨ went to the

dance. (6) So, all members of ΚΚΨ went to the dance.

(3) Gertrude is a member of ΚΚΨ, and Gertrude went to the dance. (4) Carissa is a member of ΚΚΨ, and Carissa went to the dance. (5) Elena is a member of ΚΚΨ, and Elena went to the dance. (6) So, all members of ΚΚΨ went to the dance. (7) If all members of ΚΚΨ went to the dance, then all members of ΠΡΣ

went to the dance. (8) So, all members of ΠΡΣ went to the dance. (9) Either not all members of ΠΡΣ went to the dance (some members of

ΠΡΣ did not go to the dance) or some members of ΜΜΠ boycotted the dance.

(10) So, some members of ΜΜΠ boycotted the dance.

Example of a Long Passage

Final Remarks

• Sometimes extraneous information is presented in a passage. That should be ignored.

• Like most things, constructing argument trees becomes easier with practice.

• Reasonable people can disagree regarding argument trees. Such disagreements should be occasions for reasoned discussion.