Is it immoral to convince someone of some true proposition P, by exposing them to what you know to be an unsound or invalid argument?
For example if I told my friend:
"If it rains, the grass will be wet. The grass is wet, therefore, it rained."
Now supposing it really did rain, would it be immoral to use this invalid argument to convince her?
If we answer in the affirmative, it would seem to lead to some unpleasant conclusions. For instance, it would be immoral to put a sign in my yard that says "Candidate X for City Commission", because the sign might convince people without offering them a sound argument.
But we answer negatively, it would seem to justify deception. Using unsound arguments to convince people would give them at best an unjustified true belief, not knowledge.
Is there a middle ground here?