In my previous post, I mentioned the (apparently) evidentiary argument for the intelligent design of terrestrial life. I promised to look at the fundamental flaws of this argument, but I have to digress into why we use evidentiary arguments, the inherent logical flaw in evidentiary arguments themselves, why it's pragmatically useful to work around that logical flaw rather than discard the evidentiary mode of argumentation altogether. We can then look at whether it's even possible to work around the flaw, and, if possible, the techniques that we can employ to do so.
Then we can look at how the (apparently) evidentiary argument for intelligent design fails to work around that flaw.
The fundamental flaw of all evidentiary arguments is that they elevate, at some level, a generality to a universal: From the basis that a lot of X are A we conclude that all X are A. Because some generalities are not universals, this feature of evidentiary arguments gives philosophers — even atheist philosophers such as Hume — conniption fits. Because, yes, it's not a universally valid logical operation. We can't be logically certain that the conclusion of an evidentiary argument is true — even given accurate evidence — in the same way we can be logically certain of the conclusions of deductive arguments given true premises. Since it is analytically false that some X are A entails that all X are A, we can confidently conclude that evidentiary arguments are analytically fallible: We don't even need to look at specific counterexamples to make this determination.
Since evidentiary arguments embed a principle which is not universally logically valid, no one would choose to use them except out of desperation. But we are indeed desperate. While we can be absolutely certain that logically valid operations always draw true conclusions from true premises, we have no way at all of having any idea whatsoever that our premises are indeed true. Premises are by definition not themselves logically deduced. The only reason to employ evidentiary arguments is that they are (or seem) pragmatically effective at making predictions about reality (or our subjective perceptions of reality) in a way that purely deductive arguments completely and totally fail.
If some philosopher or logician can find a way to give us interesting and pragmatically useful ways of predicting reality with deductive certainty, I'll be the first to nominate her for philosophy's equivalent of the Nobel Prize. Until then, we have to do the best we can with what we have, and try to find a way — in the absence of certainty — to make evidentiary arguments as confident as we can manage.