A subsidiary conclusion can be considered to be a premise in that it helps to support the main conclusion. However, it can also be considered to be a conclusion in and of itself.
Take the following as an example.
Premise 1: If there are no recorded typhoons in a given month, then it is impossible for a typhoon to form in that month in the future.
Premise 2: No typhoons have ever been recorded during February.
Subsidiary Conclusion: Tomorrow is February 1, so there will not be a typhoon for the next month.
Main Conclusion: Therefore, we can take a boat trip in the next month.
We can see that the subsidiary conclusion is a conclusion in that it is drawn using the support of both premises. If the main conclusion was not included, then the subsidiary conclusion would be able to stand alone as the main conclusion of the passage.
However, because there is a main conclusion, the subsidiary conclusion can also be considered to be a premise. If we do not take the subsidiary conclusion as a premise, we can see that we can no longer draw our main conclusion.
The subsidiary conclusion provides us with the essential information that there will not be a typhoon for the next month because tomorrow is February 1 and no typhoons have been recorded in February which means that it is impossible for a typhoon to form in this month. If we do not have the information that tomorrow is February 1, then we cannot conclude that it is safe to take a boat trip in the next month.
In this way, a subsidiary conclusion can be considered to be both a premise and a conclusion in and of itself.
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