How do we recognize intentional, intelligent design? How do we distinguish things that are designed from things that occur naturally? Back in the 1990s mathematician William Dembski explained that consciously or unconsciously we employ what he called an explanatory filter that helps us to immediately intuit that something is intentionally designed. What follows is a simplification of Dembski's work.
His filter consists of three steps.
When considering whether any object or event was designed we first ask whether it's the sort of thing that physical laws like the laws of electricity or gravity could've produced. If so, then it's intellectually prudent to ascribe the object or event to natural causes rather than intelligent agency. Phenomena that happen as a result of physical law have a high probability of occurring. So the first node in the filter is to ask, does this phenomenon have a high probability of occurring naturally.
If the answer is yes we impute it's occurrence to non-agential causes and rule out design.
For example, is there any physical law that makes the creation of the operating software of a computer highly probable? There doesn't seem to be, so design of the software is still a live option.
The second node is to ask whether the phenomenon has a plausible likelihood of occurring by chance. If it does then we generally attribute it to chance rather than intelligent agency.
For instance, if a poker player is dealt a royal flush (approx. 1 chance in 650,000 attempts) we'd be amazed but such good fortune can be expected to happen from time to time purely by chance, apart from any finagling by an intelligent agent. In such a case we can again rule out design.
Is it probable, though, that a poker player be dealt three consecutive royal flushes or that a computer operating system came about by chance without the input of an intelligent agent? It seems astronomically improbable.
If neither law nor chance are plausible explanations then we're left with design as the most likely alternative. Given an intelligent programmer complex arrangements of zeroes and ones that specify a meaningful computer operation are not improbable at all and given a skilled card cheat neither are three consecutive royal flushes unlikely.
We can apply the same reasoning to the origin of life and the very first DNA sequence. DNA is much like a computer program, it specifies the construction of an entire organism whether a protist or an elephant. The simplest strand of DNA, one long enough to specify a single protein necessary for the functioning of a living cell, is unimaginably complex and the probability it arose by chance makes the belief that it did akin to an extraordinary act of blind faith.
There's no law that governs the formation of DNA and the chance formation of a sufficiently long strand of meaningful DNA, a strand that specifies a necessary protein, is so improbable as to be well beyond plausibility.
Thus, the alternative that DNA was designed by an intelligent mind is the default. The only reason anyone has for refusing to accept that alternative is that they have an a priori commitment to naturalism which rules out the existence of any mind that can't be explained in terms of natural, material causes.
In other words, the rejection of an intelligent designer is an act of blind faith in the power of nature to accomplish the equivalent of writing Windows 10 purely by chance not just once but numerous times.
The following video elaborates on a simple illustration of the design filter:
