Formal and Informal Logic

It would be an error to judge all informal reasoning as either valid or invalid. Some is valid, some is not. Some informal arguemtns are equivalent to a formal logical argument, differing only in presentation. Others may be completely specious.Formal logic has advantages of clarity and rigour. A valid argument in formal logic can only be challenged on it's premises.Informal logic is less rigourous. Sometimes this is benign. So long as the argument is not represetned as a logical proof when it i s not logically valid the argument may still be strong and worth taking note of. Arguments from authority - when an appropriate authority is referenced fall into this category.Sometimes it is less benign. Crucial assumptions may be left out, argument s that have no value other than rhetorical effectiveness may be used, errors may go unnoticed.As pointed out at the time, the referenced post contains an error of reasoning. The argument is meant to establish the non-existence of "God", yet an objectio n is predicated on the non-existence of God. Such an objection can carry no weight since it denies the propostion it seeks to defend. However,as later discussion carried on the objection was refined to a valid point - that the original argument made the unstated assumption of an objective morality.Thus it illustrates some of the pitfalls of informal reasoning that might have been avoided had the argument been expressed in a more formal way.(For those interested in the basics of formal logic, ther e is my post on the subject here or a similar - but better - post at Good Math, Bad Math. Other entries in that blog have more detail for those who are truly interested)
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Jazz's point is that you are substituting your own judgements for the actual rules of logic, which does add an element of subjectivity. That is OK so long as you know you are doing it and realise that what you are presenting may not be a fully logical argument. B