The Weak Anthropic Principle
The argument from fine-tuning is the argument that given that the universe could have been any one of many possible ways, and that the vast majority of these would not have allowed the development and sustenance of life, the fact that the universe is such as to allow the development and sustenance of life requires explanation. The only adequate explanation of this fact, the argument suggests, is that the universe was created by an intelligent designer with life in mind.
One attempt to respond to this argument invokes the weak anthropic principle. The weak anthropic principle states that the ways that the universe might be observed to be is limited by the fact that observation requires the existence of observers. It is impossible to observe a universe that does not permit the existence of observers; only a universe that permits the existence of observers could be observed.
The criticism of the argument from fine-tuning based on the weak anthropic principle seeks to exploit this idea that the universe could not have been observed to be any of the ways that would not have allowed the development and sustenance of life. All possible observed universes are universes inhabited by observers, and so all possible observed universes are universes that permit life. There is therefore no need, the criticism concludes, to explain the fact that the universe is observed to be such as to permit life; it couldn‘t have been observed to be any other way.
In response to this objection, defenders of the argument from fine-tuning often make use of a story involving a firing-squad devised by John Leslie. You are to be executed by a firing-squad of a hundred trained marksmen, the story goes. You hear the command to open fire, and the sound of the guns, and then silence; you are not dead, you hear silence. All of the marksmen missed! Pondering, you realise that had the marksmen not missed you would not have been able to reflect on the attempted execution, that only a failed execution would have allowed you to be here now, listening to the silence. However, you do not infer from this that the fact that the marksmen missed is unsurprising. You remain astonished that one hundred trained marksmen could all miss simultaneously.
In this illustration, it seems that what is surprising is not that looking back at the execution you see that it failed, but that you are able to look back at the execution at all. Similarly, what is surprising about the universe is not that we observe it to be such as to allow the development and sustenance of life, but that we are able to observe it at all. The vast majority of possible universes would not allow the existence of universes, and yet the actual universe does. The weak anthropic principle does not seem to make this unsurprising any more than it does the hundred marksmen missing.