We all have secret futures, and stories never told. I could have zigged, but instead I zagged. Turned left instead of right. What if I never tried to hold her hand? Believe it or not, our understanding of how medicines work is rooted in counterfactuals like these. Imagine two universes, completely identical, except in one you were treated and in the other you weren’t. The true effect of the treatment is then the difference in outcomes observed in the two universes. It is hopefully obvious to you that this treatment effect is not something we can actually ever know, since it’s no more possible to go back in time to change your treatment as it is to go back and zig instead of zag. The past is lost to us, and there is but one universe. But this is exactly the promise of personalized medicine, a promise that nobody can keep, no matter how many times they make it. Please let me explain.
Great explanation of fundamental issues in personalized medicine!