Basic definition is flawed

As you suggest, a basic statement such as "If the premises are true, then the conclusion must be true." is just sloppy.  It’s a half truth and an example of something that appears commonly.  A proponent will make a statement that, in itself, is correct even if incomplete.  Then, the other party is encouraged to accept it as complete.  The statement is a necessary condition – I.e., if not true the conclusion isn’t supported.  At the same time, it’s not sufficient – I.e., not enough to make the conclusion true.

A more complete description might be to describe the argument as a syllogism — a three-step method of framing an argument.  At the start is a “major premise​“ — something taken as fact. Next comes the ​“minor premise”, something building on the major premise and again accepted as true.  

Then, a conclusion​ is drawn from blending the major and minor premises.

Major: from A we get to B

Minor: from B we get to C

Conckusion: therefore, from A we can get to C.

Two conditions are needed for the conclusion to be valid:

1) premises must be true, and

2) premises must be linked and must lead to the conclusion.

just saying premises need to be true gets us part way there; but the links need to be there, too.

You are correct. The premises being true is only one half of the equation, the other half is that the conclusion must be logically drawn from the premises. An argument which lacks the second half is a logical fallacy. The example you provided is a pretty classic example of a non sequitur . The premises are true, but they have nothing to do with the conclusion, therefore they do not prove the conclusion.

Hi Reverend Jim!

Your Question informs us that when Dillahunty says “if the premises are true, then the conclusion must be true”, he is doing so in an attempt to provide us with a definition of a certain word. Very unhelpfully, your Question leaves out any clarity on what the word is which Dillahunty was attempting to define. However, Dr. Bennett suggests that Dillahunty is attempting to define the word “valid.” In which case, Dillahunty is right and excellent, not flawed, not sloppy, not false or misleading. A basic definition of a valid argument is in fact “if the premises are true, then the conclusion must be true.” 

Some people here who Answered your question seem to think that Dillahunty’s statement was meant to be a blanket description about all arguments or all syllogisms. However, there is no context to support that thought. Your Question, too, seems to make the mistake of assuming that Dillahunty is defining the word “argument.”

Thank you, Reverend Jim.

From, Kaiden

You (or he) is missing an important part:

“ For the argument to be valid, if the premises are true, the conclusion must be true.”

This is true for deductive arguments.