Hello, Farkel
You write:
All blonds are sexy.
Suzie is not sexy.
Now from these premises, we can deduce that Suzie is not a blond, but I don't see how that conclusion is CONTAINED in those premises. I can see how it is DERIVED, though. Circular arguments on the otherhand STATE the conclusion in the premises. I think we are still on the same page though, because if there was never enough evidence to derive a conclusion there would be no such thing as deductive reasoning.
That Susie is not blonde is contained in the first premise because to know all blondes are sexy would mean all blondes had been observed and Suzie would not have been observed as a blonde, which means to know the first premise is to know Suzie is not blonde. This mean that, for the first premise to have any veracity would mean that the conclusion that Suzie is not blonde was already known. In the minds of some past logicians this made the syllogism worthless as a means of proving a conclusion. The concern was the syllogism was reduced to saying what is already known, which would make it seem like a dressed up version of begging the question.
Here is another example:
1) All bachelorettes are unmarried
2) Mary is a bachelorette
3) Mary is unmarried
This syllogism has a major premise we do not need to observe since by definition bachelorettes are unmarried. But there is still a degree of circularity in the reasoning because the only way we could know Mary is a bachelorette (the second premise!) is to first know Mary is unmarried. If we did not know Mary is unmarried then we could not possibly know Mary is a bachelorette. This means the entire syllogism is no more than an exercise to tell what must already be known to tell it.
I am not sharing this information to infer that deductive argumentation is worthless or anything but precise in how it functions and helps us. I am only sharing part of the philosophical thinking that has led to what we know and use today as deductive logic. This history does not in any way diminish deductive logic but rather knowing it and understanding its resolution only strengthens the confidence we should have in this form of logical thinking.
The real value of deductive logical form, and the resolution of the problem above, is that it demonstrates something we may not have noticed otherwise. I emphasize "may" because conclusions are contained in the premises of categorical syllogism. Nevertheless, deductive reasoning helps us because it does just what it says, it deduces. In the case of deductive reasoning we can deduce what is already a fact but has went unnoticed until now to us. But when we use deductive logic we are most often using it to help someone else see what is a fact but has went unnoticed to them, for whatever reason. This is why we do not use deductive logic to convince ourselves of what we already know (circular at its worst!) but rather to build awareness of facts we can deduce from what we already know. Of course we also use deductive thinking to teach others.
A bonus feature of deductive logic is its conclusiveness.
Marvin Shilmer
