I just checked with two practicing PhD physicists who both liked my analogy about degrees of understanding. One said: "I think it's a good analogy, in that different kinds and levels of understanding are useful in different contexts. " The other commented: "We’ve done a good job over the last few centuries at developing an understanding the rules, but the process by which the rules were written remains unknown. In that sense, I would say we don’t understand gravity, but it certainly doesn’t stop us from predicting the existence of black holes, or gravitational waves (predictions that were subsequently confirmed)."

So we are not debating whether or not we "have" understanding, we are debating our comfort level over our degree of understanding regarding how antidepressants work.