Editors' Note: This response from epidemiologist John Ioannidis is part of an exchange on an essay by the philosopher of medicine Jonathan Fuller. Read also the response by the epidemiologist Marc Lipsitch.

A person suddenly collapses on the floor—what do you do? Given the choice between acting or not acting, surely every reasonable person will say we need to act without hesitation.

But how? We first quickly collect the available data: we check whether the collapsed person has a pulse, whether he's breathing, whether he responds to verbal cues. If not, we suspect cardiac arrest and immediately start CPR—but still we try to collect both new and better data as we go along. If a blood pressure monitor becomes available and we find the patient's pressure is fine and his pulse is regular—though we didn't even feel one at first—everything changes; the situation is not as dire as we had thought. Perhaps he begins talking, though still his breathing is labored: our chest compressions have broken his ribs.