An atheist friend and I (I am a theist) had a long series of discussions about the existence of god, and his comments made quite an impression on me. I found what he said so stimulating, in fact, that I beagn to read more philosophy of religion to help me better understand the nature of the issues raised. One question, however, is a bit puzzling, and I have not read much about it, though I have seen it raised in atheist/theist debates about the existence of god. The issue is simply falsifiabilty: how can we know if some occurrence of anything is an act of god and therefore, say, the result of prayer, or the result or effect of natural processes? For example, if I pray for a sick relative and she recovers, I can say god healed her; but I can also rightly argue that medical science healed her; or, even more precisely, physicians using medical knowledge stabilized her body so that it could heal itself. I know many theists regularly thank god for certain acts (many of which they pray for) that could easily be explained in more natural terms. How do we know when an act can rightly be attributed to god, and therefore the result of prayer, and when one can be attributed to something quite natural? I have considered various ideas (e.g. things one prays for that are congruent with some religious text are caused by god; things that are congruent with god's nature, i.e. good things, are caused by god; etc.), the answers I have considered seem problematic in one way or another.