Answer:  only if it’s good economic theory.

Any theory, not just economics has this structure: I assume A, I conclude C. It’s a good theory only if A logically implies C. And if A implies C then assuming A entails assuming C. Observations:

  1. If someone is not assuming their conclusion then you should ask them to come up with a better theory.
  2. Assuming your conclusions is a necessary condition for a good theory.  It is not sufficient:  it is possible (typical?) to assume your conclusion but have a bad theory.
  3. Once we have established that C follows from A, we can do the real work of evaluating a theory:  assessing whether we believe A.
  4. That is the beauty of economic theory and other parts of the social sciences where we assume our conclusions:  you get to see exactly what to focus on in trying to evaluate the theory.  A theorist’s job is to take an argument and decompose it into two parts: the rules of logic and the assumptions.  You can save your time not trying to evaluate the first part because you know in advance how logic works.  We put in the hard work of separating that out so that you can see what’s left to argue about.
  5. There is no point in trying to figure out if someone has “predetermined their conclusion and picked assumptions that imply it.” The timing of how the theory came into existence is irrelevant.  If it is a good theory and the theorist assumed his conclusion then you get to see exactly what assumptions led him to it.  You get to decide whether you agree with C by deciding whether you accept A. And you are given a roadmap for how to convince the theorist he is wrong.
  6. In fact, given that we all have predetermined conclusions I would rather argue with someone who makes up a set of assumptions that imply his predetermined conclusion than someone who doesn’t.  The first is doing the honorable thing of setting out conditions under which he can be proven wrong.  There is no way to get started debating with the second person.