Some of those prone to seek incontrovertible evidence for an intelligent (and presumably divine) designer of the cosmos have fastened on to the obvious fact that some key physical attributes of our world, and indeed of our entire cosmos, have to be exactly as they are for us to exist. If any one of these factors (like the distance of earth from the sun, or the energy flow of the sun, or a number of physical constants) were even a tiny bit different, we wouldn’t be here. (see for example Just Six Numbers by Martin Rees, 1999) This they find as evidence that we exist because some intelligent force must have “tuned” the cosmos to be just right for us to exist.
Appealing as this argument is, it is fatally flawed, because the only possible universe we could study would be one in which we exist, so any universe we do study will of course be “tuned” for us to exist (or rather, we will be “tuned” to survive in it). There may well be many other universes (before ours, after ours, and/or existing simultaneously with ours) in which the parameters wouldn’t allow us to exist, but of course we can never observe those because we can’t exist in them to observe them.
It’s a little like being amazed that we always find things in the very last place we look. If we didn’t find our lost item in the last place we looked, we would keep looking, so it wouldn’t be the last place. Just so, if the critical constants in the universe weren’t such that we could exist, we wouldn’t be here to observe it.
Now this may all sound a bit arcane, but it is an example of the sort of subtle reasoning error into which we humans can fall, and understanding it ought to help to keep us from being so certain we are always right.
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