OK, so here we have a clear majority of voters who think the argument is valid. Those who think the argument is not valid haven't properly explained why.

One crucial point about this argument is that it is notformallyvalid.

That the argument is not formally valid can be seen by the fact that its validity doesn't depend only on the explicit form of the argument.

Specifically, to understand the argument as valid, you need to assume what the three non-logical terms mean: "female", "sibling, "sister".Everyone is female;

Therefore, any siblings are sisters.

Bomb#20 has been kind enough to provide a convincing interpretation of these terms whereby the argument is not valid.

Please note that professional logicians, however incompetent I think they are, nonetheless usually accept that this argument is valid even though they would all say that it is not formally valid. The distinction is indeed often part of the way logical validity is explained in textbooks.

This shows that, contrary to what fast argued, the notion of validity simpliciter, as opposed to formal validity, is accepted by professional logicians as our ordinary, layman notion of the logical validity of an argument, as it is demonstrated here by the answers of most posters.

Thus fast wrongly argued that validity is a technical term in the context of logic when in fact the proper technical term is "formal validity".

Of course, mathematicians themselves will usually use the term "validity" rather than the technical term "formal validity" when they in fact mean formal validity, thus confusing fast as to which is the technical term.

So, will fast now blame mathematicians for using the non-technical term "validity" in lieu of the technical term "formal validity", thereby confusing his judgement?

The argument above could be made formally valid by making explicit our implicit assumptions about what the non-logical vocabulary means:

It is noteworthy that most people would find it more difficult to assess the validity of the formalised argument, essentially because it is more complicated and wordy, and this even though this is essentially the same argument. This falsifies the idiotic claim that people are confused about validity. People will undoubtedly be confused about formal arguments given that most people don't have the proper training to read them, but they are not confused about validity, as demonstrated by the answers given here.p1 - For all x, Sister(x) implies Female(x);

p2 - For all y, Sibling(y) implies either Sister(y) or Brother(y);

p3 - For all z, Female(z);

C - Therefore, for any a, Sibling(a) implies Sister(a).

EB