To understand the logical structure of an argument is simply a matter of knowing what these three components are. To determine what the logical structure is one needs, therefore, to answer the following questions:
(1) What is the basic conclusion that this argument is attempting to establish?
(2) What are the premises, or assumptions, that the person is putting forward in support of the conclusion?
(3) What are the inferences that the person is making, and which are supposed to take one from the premises to the final conclusion?
How does one go about answering these questions? The answer is that a passage that contains an argument will generally contain a number of words or phrases that function as inference-indicators. Consider, for example, the following sentence:
"I have just polished off two six packs; I am feeling very nauseous; I am unable to get up off the floor, and the rest of the world is spinning around me at something approaching the speed of light. Therefore I am probably slightly drunk."Here the word "therefore" is an inference indicator, and it functions to indicate that the fact that I have just polished off two six packs, am feeling very nauseous, am unable to get up off the floor, etc., is a reason for drawing the conclusion that I am probably slightly drunk. So the pattern it points to is the following:
[Reason, inference indicator, conclusion].
Other inference indicators work in the opposite way. They indicate that what follows the inference indicator provides a reason for what precedes it. Here is an example of that sort of inference indicator:Here is an example of that sort of inference indicator:
"Mary is probably a better tennis player than I am, since she has beaten me 6-0 in each of the last ten sets we have played."The word "since" is an inference indicator. In particular, it indicates that what follows it - i.e., the fact Mary has beaten me 6-0 in the last ten sets that we've played - is a reason for believing what precedes it - i.e. that Mary is probably a better tennis player than I am. So the pattern this sort of inference indicator points to is as follows:
[Conclusion, inference indicator, reason].
Notice, however, that with words that are inference indicators of this second type, it is sometimes possible for the world order to be inverted:
"Since Mary has beaten me 6-0 in each of the last ten sets we have played, she is probably a better tennis player than I am."When this is done, the pattern that this sort of inference indicator points to is instead this:
[Inference indicator, reason, conclusion].
But what is always true with this sort of inference indicator is that the inference indicator is always followed by the reason.
therefore
hence
consequently
so
entails
implies
from which it follows
2. Inference Indicators of Type 2: [Conclusion, Inference Indicator, Reason]
[Inference Indicator, Reason, Conclusion]
since
for
because
is implied by
is entailed by
is a consequence of
follows from
The basic idea, then, is that one can pick out passages that contain arguments, and begin to work out the logical structure of the argument, by looking for words and phrases such, as the above, that function as one or other of the two types of inference-indicators. In doing this, however, there two points that it is important to be note. First. there are other words and phrases, besides those listed above, that function as inference-indicators. The above lists contain only some of the more common ones.
Secondly, some words that sometimes function as inference-indicators do not always do so. Thus, for example, the word "because" in the sentence, "Suzanne has been swimming very good times because she has been doing a good deal of weight training", does not function as an inference-indicator, since the fact that Suzanne has been doing a good deal of weight training is not being offered as a reason for believing that Suzanne has been swimming very good times. It is being offered, rather, as a causal explanation of why she is swimming good times. So it is important to distinguish, in the case of sentences containing the word "because", sentences that offer reasons for thinking that some claim is true and sentences that offer a causal explanation (or other type of explanation) of why something is the case.
As another illustration of the need for care in identifying inference-indictors, consider the following three sentences containing the word "since":
(1) 1001 is not a prime number, since it is divisible by 11;
(2) Paul hasn't written to me since he went to Europe;
(3) Since Laura is living in Alaska, she no longer plays golf all year round.
In the first of these sentences, "since" does function as an inference-indicator, but in the second it indicates, instead, a temporal relation, while in the third it refers to an explanation of a certain fact, rather than as a reason for thinking that something is true.