Ways That Premises Support Conclusions

· By Linking – Linked premises can support the conclusion only when they are taken together.

P1) Vunerability to heart disease is either inherited or environmental

P2) Vunerability to heart disease is not environmental

C) Therefore, vunerability to heart disease is inherited

A or B

Not B

Therefore, A

P1) I hate anything that endangers my car

P2) the snow makes roads slippery

P3) When roads are slippery my car is in danger

C) I hate the snow

For all x, if x endangers my car then I hate x

Snow endangers my car

Therefore, I hate snow

· By converging – Convergent premises are seen as each offering independent support for the conclusion.

P1) The 1985 Chicago Bears held the single season record for least points allowed per game for 15 years.

P2) The 1985 Chicago Bears had an explosive offense featuring a proven leader at quarterback and an eventual record-setter at running back.

C) The 1985 Chicago Bears were the greatest football team in NFL history.

P1) Tobacco kills more Americans than AIDS, drugs, homicides, fires and auto accidents combined

P2) Tobacco companies make a product that kills 1,200 Americans a day.

P3) Cigarettes will eventually kill a third of the people that use them.

C) Therefore, Cigarettes are a deadly product that should be banned in the United States

More Student Examples

P1) Living on campus is costly

P2) Living off campus has many benefits.

P3) Apartments have many conveniences that the dorms don’t offer

C) It is more affordable and enjoyable to live in an apartment off campus than one on campus.

P1) Athletes spend too much time with sports and not enough time with academics

C) Athletes are not only jocks, but are perceived as jocks only by non-athletes.

P1) President Bush was not elected by the majority of the people.

P2) Under President Bush the economy has gone into recession.

C) President Bush should not be re-elected.

The Problem of Missing Premises

“In fact, the ordinary orange is a miniature chemical factory. And the good old potato contains arsenic among its ore than 150 ingredients. This doesn’t mean natural foods are dangerous. If they were, they wouldn’t be on the market.”

P1) If natural foods such as oranges and potatoes were dangerous, they wouldn’t be on the market

C) Therefore, natural foods such as oranges and potatoes are not dangerous.

Missing Premise: Oranges and potatoes are on the market

P1) Bill Clinton committed perjury

C) Therefore, Bill Clinton should be impeached

Missing Premise: If Bill Clinton committed perjury, then he should be impeached

P1) We failed to find weapons of mass destruction in Iraq

C) Therefore, the war in Iraq was not justified

Missing Premise: Only the existence of weapons of mass destruction could justify the war in Iraq.

P1) The Sun has risen every morning for the past several billion years

C) Therefore, the sun will rise tomorrow.

Missing Premise: The future will resemble the past.

Principle of Charity: When extracting an argument from a text try to do so charitably. Avoid attributing loose reasoning and implausible claims unless there is good evidence for doing so.