What is meant by 'deductive'?

The video used another technical word — 'deductive'. Later in the course we will be distinguishing between deductive arguments and other types of argument but for now all you need to know is that

a deductive argument is one where the intention is that the premises prove the conclusion to be true.

In other words, a deductive argument is one that aims to be valid. The reason why deductive arguments are defined this way is it allows for the possibility of bad deductive arguments, that is, arguments that are badly structured so that the premises don't guarantee the conclusion. These arguments are said to be invalid.

The strict definition of valid is:

An argument is valid if there are no possible circumstances in which the premises could be true and the conclusion false.

For most purposes it is enough to say:

An argument is valid if its structure means that if the premises were true the conclusion must be true.

You can think of this as saying:

An argument is valid if the truth of the premises guarantees the truth of the conclusion.

Remember, in its technical sense:

  • The word ‘valid’ only applies to arguments; it doesn’t apply to statements.
  • An argument is either valid or invalid; it is not a matter of degree.
  • Whether an argument is valid or invalid depends on the structure of the argument not on whether the premises and conclusion are actually true
  • The words ‘valid’ and ‘invalid’ are normally used when commenting on deductive arguments.

Deductive validity isn’t everything. As already mentioned, there are other kinds of arguments that we will be looking at that don't set out to guarantee the truth of the conclusion but are still accepted as perfectly good arguments. For example,

Every winter in recorded history it has snowed somewhere in Scotland therefore we can be sure it will snow somewhere in Scotland this winter.

If this was judged as a deductive argument it would be invalid but it is more reasonable to say that this argument was never meant to be a deductive argument, i.e. there never was any intention that the premise would guarantee with the conclusion with the kind of certainty offered by deductive arguments. In other words this is one of those non-deductive arguments and the question of validity or invalidity doesn't really apply.