More than 30 years ago, the older of us published a paper with the proposal that all scientific papers should start with a statement along the lines of: "These are the opinions on which I base my facts". Why pretend? To take a topical example, if you are on the nature side of the nature/nurture debates, is it likely that your next paper will be an apologia: "I take it all back; genes don't matter at all; it is all environment"? Unlikely. Similarly, if you are on the other side. (We know. It's both.) Here, we are not in any way arguing for a relativist credo that would say opinions are all. Along with other readers of this journal, we spend a good part of our working lives gathering, analysing and interpreting empirical evidence. Evidence matters. But everyone has values and they do affect our positions.