Just recently saw the following argument in a logic book: all lions are herbivores all zebras are lions ————– therefore all zebras are herbivores this seems to be logically valid syllogism, but it is disturbing.

I have been reading your site, and there is somewhere you said a conclusion can logical valid but unsound. Is the following argument valid but unsound? I am not sure about what unsound arguments mean? Can you please clarify this for me. – Johnson Mafoko

Yes, that is a valid, unsound argument. The structure is good, but the content is bad. This is the case even though the conclusion is correct.

The way it works is this:

Invalid means the structure is bad. There are no benefits to an invalid argument, the premises have no meaningful connection to the conclusion.

Valid means the structure is good. If an argument is valid, it means the conclusion is at least as good as the premises. So if you put in true premises, you get a true conclusion. However, it doesn’t mean that if you put in false premises you get a false conclusion. In logic, false premises can lead to any conclusion, even when the argument structure is valid.

Sound means that the argument is valid and that the premises are true. A sound argument will guarantee a true conclusion. It is the only type of argument that guarantees a true conclusion.

Please note that only “formal” arguments –the kind of very artificial, highly structured arguments found in logic books and dealing only with unambiguously true or false statements –can be either valid or sound. (Different terms are used for less formal arguments).

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