Even if there's no such thing as free will, we have to treat each other as if there were free will in order to live together in society. Because otherwise, every time somebody does something terrible, you can't punish him, because he can't help it, because his genes or his environment or God made him do it, and every time somebody does something good, you can't honour him because he was a puppet, too. If you think that everybody around you is a puppet why bother talking to them at all Why even try to plan anything or create anything, since everything you plan or create or desire or dream of is just acting out the script your puppeteer built into you.