Sir Iain Chalmers, the founder of the Cochrane Collaboration, and Jeremy Howick invited me to give a lecture in 2019 at an MSc course in Oxford about the history and philosophy of evidence-based healthcare. They asked me to share my personal experiences and research on conflicts of interest under the heading, “Power versus rationality.” In other words, it was about speaking truth to power. As this subject might have general interest, I have summarised my lecture here.