Do you think that the following argument is a good one : X says that a certain metaphysical view is correct. X says that if this view M is correct, it should produce certain effects in the psychology of people that realize it. (Make them super-smart, or super-kind for example). It is observed that X has this type of psychology. Thus the metaphysical view M is true.

I am part of a minority of philosophers who believe that some variant on the argument you proposed can be incorporated into a strong inductive argument –in fact I’m currently at work on just such a project, named “Pragmatic Metaphysics”. However, the argument as so stated is an example of the formal fallacy “affirming the consequent”.

P1. If Metaphysical Claim then I am Super Kind
P2. I am Super Kind
C. Therefore Metaphysical Claim

One problem is that you haven’t explained why we believe P1 is true, But this argument fails even if P1 is true because we have never established “If Super Kind then Metaphysical Claim”, and the relationship is not reversible.

The danger with affirming the consequent is this: If you start with a true and known statement “B”, such as “2 + 2 = 4”, then every conditional of the form “If A then B” will automatically be true, regardless of the nature of “A”. Thus, “If it is sunny tomorrow, then 2 + 2 will equal 4.” “If it is not sunny tomorrow, then 2 + 2 will equal 4.” “If the Yankees win the World Series then 2 + 2 will equal 4.” “If the devil beats his wife, then 2 + 2 will equal 4.”

Because of this, the type of argument you outlined can never be formally sound. However, it is possible to make a similar strong inductive argument, given the following conditions:

  • “A” (i.e. Metaphysical Claim) must be a reasonable explanation for many observable things, not just “B” (Super Kind). So “If A then B, C, D, E, & F”, where B through F are all true.
  • “A” must not have further implications that are false. So it must not be the case that “If A then G” where G is false.
  • “A” must have predictive value. So “If A then H tomorrow,” where the prediction comes before H is observed, but where H confirms the prediction.
  • “A” must not have stronger rivals. There must not be an A´ or an A´´ that explains the same data in a simpler and better fashion.

Even given all this, we do not prove “A,” we can only say that there is reason to believe A, or that A offers good explanatory value. For instance, if we say it is a natural law all planets move in ellipses, there is as sense in which this is a metaphysical claim (since we can observe the planets but not the law in itself). We accept it as true because it has many true implications, no important false implications, excellent predictive value, and no stronger rivals.