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?

Your question raises some fascinating issues. I think it will help to separate your question into two distinct concerns: (1) Is it immoral to use faulty reasoning to convince someone to believe something? (2) Is it immoral to place people in a situation where they might believe something on the basis of faulty reasoning? My first instinct is that situations involving (1) are likely to be immoral, whereas those involved in (2) are probably not. In cases of the first sort such as cases where you use poor reasoning to convince someone of something, as Professor Smith notes, there is a degree of deception at work. While most moral philosophers don’t think that deception is always a bad thing, they nonetheless think it is bad absent special justification. We can imagine cases where deception is harmless (as in your example, of using faulty reasoning to get someone to believe it rained, when in fact it did rain), or even beneficial (as could happen when you deceive someone to do something that is good for...