We teachers want to teach the "truth" or reality. What is reality? First, there is a religious reality, which cannot be proven or disproven. I think it exists but I have no means other than argument to convince. The other reality is a logical, scientific reality based on proving a hypothesis. Hypotheses are checked using replicated experimental studies. These can be proven at a certain confidence level, typically 95 percent, meaning that we have a 95 percent certainty that the change in the dependent variable is due to the intervention and not to chance. This is how we know, through experiments, that medical or other interventions do certain things.
We also have correlational studies, but these are less convincing because variables are not controlled, so a third variable could be the cause. Anecdotal evidence is similar. It has emotional weight, and we really believe it when we see it in front of our eyes, but it is not logically convincing because another variable could be the cause. For example, a kid gets sick after a vaccine. It could be the vaccine, but it could also be thousands of other things. Only the scientific method, experiments, will find the truth. So, not all evidence has equal weight or validity. Otherwise, reality would be a matter of opinion and personal choice (as it is in the religious realm).
In psychology, biology, physics, and chemistry classes we test reality through experimentation as described above. As a history teacher, how do I teach reality? We do not perform or cite experimental studies in history. All of the evidence is correlational at best and usually anecdotal. So how do we know if the Democratic party ensures greater prosperity or the Republicans defend the country better? How do we know if Keynesian policies work? We cannot be as sure that past successes or failures were due to this or that policy, though if the same results keep happening I would form biases. Since we don't really know the truth, the only fair way to teach non-scientific controversial issues is to give our students information about all the major viewpoints.
