Two dimensions of rationality: "Isn't SF based on constructivism? I may be too radical, but in this case that would mean that there's no such a thing as "what's true" (more precisely: what's true is unknowable for any kind of knowledge is constructed into one's own mind). So, it boils down to "what works" and "what you experienced as true" (meaning that someone else may have experienced something differently, being "true" on different points or even "false" from their point of view)."
My response is: "Thank you for your reaction. Yes, one of the main inspirations for the people who originally formulated SF was social constructivism / social constructionism, which were popular philosophical perspectives in those days. These knowledge theories consider how social phenomena or objects of consciousness develop in social contexts. Some people, inspired by these ideas think that objective reality is not knowable for us. Others have even said that objective reality does not exist. The latter people may say that there is not one truth but there are many truths or they may reject the whole notion of truth and reality.
Personally, I think that rejection of the notion of truth and reality is too radical and unconvincing. Take the example of the shape of the earth. Once, people thought it to be flat and they turned out to be wrong. Later, they thought it was spherical which turned out to be wrong too (it only approximates a sphere). Can we conclude from this that whatever we think will always be overturned later and that truth really does not exist? I think this is a wrong, even dangerous, conclusion. The claim that the earth is flat is much more wrong than the claim that it is a sphere. To say that the earth is a sphere approaches reality much closer than to say it is flat as can be empirically tested. If we say that all claims are equivalent, there would be no measure of progress anymore, there would be no need for evidence and science anymore, and the whole notion of education would collapse. To deny that one claim could be further from the truth than another would be to say that anything goes.
People, of course, have different perspectives, values, viewpoints etc. They may call this 'their truth'. While I acknowledge that we all have different perspectives and while I agree that this is hugely important, I think it is also important to acknowledge that there is a reality which is independent from our perspective. For instance, one person may say the earth is 6000 years old, another may say it's 4.5 billion years old. These perspectives may seem equally true but they aren't. An example from a different domain: if an individual develops his own cancer cure, does no research, yet 'feels' and claims it is the most effective cure, this person, in my view, should not be allowed to practice it, even if it seems true that person and his followers.
While we may never be able to formulate definite descriptions of reality/truth, we can certainly distinguish between false and falser. We can certainly make progress in finding out what is true and what not. I think this is not only relevant for astrophysics and other 'hard' sciences but also for psychology, conversations, cooperation, etc. For instance, I believe, there are useful answers to be found to questions about what supports human thriving and what thwarts it.
Although I am not rejecting the existence of truth and objective reality, I am not any less enthusiastic about SF. Through taking the perspective of my client as the basis for our conversation and by asking questions which are directed at his/her preferred future, I do not at all have to discuss with him or her what is true or what is not true (let alone that I would need to convince him or her of what is true). The SF process works in the sense that it helps people become more realistic and effective. But this does not mean, as far as I am concerned that reality and truth do not exist.