Topics Logic

Question In a critical thinking textbook I’m trying to study from, there is an exercise which gives groups of three different independent reasons from which I must select the one which supports a stated conclusion. For example: Conclusion: Blood donors should be paid for giving blood. (a) The blood donor service is expensive to administer. (b) People who give blood usually do so because they want to help others. (c) There is a shortage of blood donors, and payment would encourage more people to become donors. (Anne Thomson, <i>Critical Reasoning - a practical introduction</i>.) For each question I must pick the answer which could be a reason for a conclusion, say why it is the right answer, and why the other options are wrong. I’ve had absolutely no problems selecting the correct answer, but I can’t seem to say why. It would seem that I could easily say THAT a particular reason gives or doesn’t give support to a conclusion, but I can’t seem to put into words HOW or WHY. So my question is, why and how do reasons support conclusions? Or what does ‘support’ or ‘gives us reason to believe’ mean? Or why and how do conclusions 'follow' from reasons? Thanks.

Accepted:
November 15, 2006

Comments

In the most straightforward case, that of deductive inference, reasons support a conclusion in this sense: if the reasons are true then the conclusion must be as well. Once one moves beyond deductive inference, the truth of a good argument's premises makes the truth of its conclusion more or less probable. If you're asking the question "How does it come to be that the truth of some claims makes the truth of others either necessary or highly probable," that's a much contested issue that haunts the philosophy of logic, the philosophy of language, and the philosophy of science.

One way of further spelling out Alex's standard for deductive inference ("if the reasons are true then the conclusion must be as well") is to use the idea of "possible worlds"--different ways that the world might be. To say that if the reasons are true then the conclusion must also be true means that in all possible worlds in which the reasons are true, the conclusion is true.

The idea of possible worlds can be helpful in thinking about many kinds of reasoning. For example, consider the problem you presented about blood donors. One way to think about the problem is to consider different possible worlds. What would the world be like if blood donors were paid? What would the world be like if they weren't? Choice (c) says that in a world in which blood donors are paid there would be less of a shortage of blood donors than in a world in which they aren't, so it supports the conclusion. The other two choices don't give any sense in which a world in which blood donors are paid would be better than one in which they aren't.