- Mon Jan 25, 2021 2:13 pm
The idea here is that if all we have in the premises are opinions, they will not support a fact-based conclusion, and vice versa. You cannot use evidence about what is good or what should be true to prove what will happen or must happen. You cannot use evidence about what must or will occur to prove what should occur. It's not a question of whether the opinions are "true," but about what kind of things you can and cannot prove based on those opinions.
Here's another example:
People ought to treat their neighbors with kindness and respect. Betty and John are neighbors, so Betty is obviously kind to John.
The premise here is an opinion about what ought to be the case. The conclusion is a fact that is not proven by that opinion. Maybe Betty ought to be kind, but that doesn't prove that she is kind. Maybe she doesn't do what this author thinks she ought to do?
Or try this one:
You prefer pizza to tacos, so since you are hungry right now you will certainly get a pizza instead of tacos.
Again, just because you have a preference (which is a form of an opinion) I cannot use that to prove what action you will take. Maybe there is no pizza available, but you are standing in front of a taco truck? Maybe you love tacos, and despite your general preference for pizza you will choose tacos on this occasion?
Match the type of evidence to the nature of the conclusion, as well as matching the strength of the evidence to the strength of the conclusion. You cannot use evidence about what could occur to prove what will probably occur or what will certainly occur. Just because you could get a pizza doesn't mean you probably will, or that you certainly will.
Adam M. Tyson
PowerScore LSAT, GRE, ACT and SAT Instructor
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