Hi lsat2016,
Thanks for your questions.
First, let's get our terminology straight: in logic, we don't speak of arguments as being "true" or "false," nor do we speak of statements as being "valid" or "invalid." On the LSAT, we assume that the premises upon which the author's conclusion is based are true statements. There are very, very few instances in which test makers would ever let us question the veracity, or the truth value, of a premise.
Assuming that all the premises are true, there are two categorically different types of arguments you'll come across on the test:
deductive and
inductive arguments. How we evaluate their conclusions, and the labels we use in our assessment, will depend on the type of argument we're dealing with.
Deductive Arguments
In a deductive argument, the conclusion is supposed to
necessarily follow from the premises. If it does, then the argument is considered "valid." However, if it is possible for the conclusion to be false despite the truth of the premises, then the argument is invalid. There is no middle ground here: deductive arguments are either valid or invalid. For instance:
Paris is the capital of France. I live in Paris. Therefore, I live in France.
This argument is valid, because the conclusion necessarily follows from the premises.
Paris is the capital of France. I live in France. Therefore, I live in Paris.
This argument is invalid (it commits a Mistaken Reversal). The conclusion could be false, because I could live outside Paris and still live in France.
Note that the conclusion in invalid arguments can still be "true," factually speaking: validity only refers to the extent to which the premises support, or do not support, the conclusion! Validity has nothing to do with whether the premises are true or false; we assume that they are true. Rather, validity is determined by the relationship between the premises and the conclusion.
- The only question you need to ask yourself when evaluating deductive arguments is this: do the premises fully support the conclusion? If they do, then the argument is deductively valid. If not, then the argument is invalid.
Argument forms associated with deductive arguments include:
- Conditional Reasoning
Formal Logic
Arguments based on mathematics (where the conclusion depends on purely mathematical computation)
(Note: people sometimes speak of "sound" and "unsound" deductive arguments. "Soundness" is not synonymous with "validity." A sound deductive argument is a deductive argument that is valid and has all true premises. Since the premises on LSAT arguments are assumed to be true by default, the distinction between "soundness" and "validity" is a moot point. On the LSAT, all valid arguments are sound arguments).
Inductive Arguments
An inductive argument rests on probabilistic reasoning: the author must argue that the conclusion probably, or likely, follows from the premises. The conclusion of an inductive arguments often goes beyond the scope of the premises, which is fine, as long as the language of the conclusion is carefully qualified (by words such as "probably" and "likely").
Depending on the actual strength of the inferential link between the premises and the conclusion, such arguments are classified as either "strong" or "weak." A
strong inductive argument is an argument where the conclusion is probably true, given that the premises are true. Conversely, a
weak inductive argument is an argument in which the conclusion does not probably follow from the premises. Evaluating the strength of these arguments depends on a number of considerations, such as the strength of the evidence presented, the availability of alternative causes (in causal arguments), etc. This is a much more complicated (and holistic) task than evaluating the validity of a deductive argument. Unlike our assessment of deductive arguments, there is middle ground here: arguments can be strong, weak, or anything in-between.
- When evaluating the strength of inductive arguments, here's the question you should keep in mind: do the premises provide probabilistic support for the conclusion?
Argument forms associated with inductive arguments include:
- Causal reasoning
Predictions
Arguments from analogy
Generalizations, incl. arguments using statistical samples
(Note: people sometimes speak of "cogent" and "uncogent" deductive arguments. "Cogency" is is the inductive equivalent of "validity." A cogent inductive argument is an inductive argument that is strong and has all true premises. Since the premises on LSAT arguments are assumed to be true by default, the distinction between "cogency" and "strength" is a moot point. On the LSAT, all strong inductive arguments are cogent).
To help you differentiate between deductive and inductive arguments, take a look at the indicator words below:
- Deductive
Inductive
certainly probably/improbably
absolutely likely/unlikely
definitely plausible/implausible
necessarily it's reasonable to conclude that...
Compare the following two arguments:
All prospective law students must take the LSAT before applying to law school. Gina has not yet taken the LSAT. Therefore, she cannot apply to law school.
All prospective law students must take the LSAT before applying to law school. Nikki has no plans to take the LSAT anytime soon. Therefore, Nikki will probably not apply to law school.
The first argument is deductive (and is also valid), as it takes the form of the contrapositive. The second argument is inductive (and is also quite strong), because if the premises are true, then it the conclusion is probably true as well.
On the LSAT, most deductive arguments will be invalid. Why? Because the premises would provide
some support for the conclusion, whereas the conclusion will be argued to necessarily follow them. Most invalid deductive arguments will be flawed because of a strictly formal type of a logical fallacy, such as the MR or the MN, or else because of an error in the use of evidence. Likewise, most inductive arguments will be quite weak: they could be using an unrepresentative sample to draw a probabilistic conclusion, commit a causal fallacy, make an appeal to authority, contain an error of equivocation, composition or division, etc. There are many types of logical fallacies out there. Depending on the type of argument we are dealing with, such fallacies would either make a deductive conclusion
invalid, or else they would make an inductive conclusion
weak.
To answer your last question, an example
is evidence (whether good or bad). Whenever an example is used to support a given conclusion, the example functions as evidence (or a premise) for that conclusion. Of course, not all premises entail the use of examples.
Hope this helps!
Thanks,
