a.k.a. You can't argue for biological (rarefied) design using examples of human (ordinary) design.
The Long Answer:
The argument by analogy is an extremely common, and if used properly, completely valid form of argumentation. However, all arguments by analogy are not necessarily necessarily valid, and there are various ways of testing an analogy. Some tests include:
(see Argument by analogy for a full discussion)
A good example for design detection that is analogous to that used with ID in biology are the stone faces on Easter Island. The example for the stone faces on Easter Island is an example of how we go about inferring design. The stones are shaped in a complex and specific way such that we recognize that they are supposed to be faces. Based on this observation, we can infer that they must have been designed. We can determine this even if we don't know who the designers were, how they exactly worked, nor what the intended purpose of the design was. This analogy works because there is complex and specified information (CSI) in the heads of easter island, just like there is CSI in irreducible complex structures found in biology. Although the form of the CSI is different, it is the presence of the CSI that matters, thus the analogy holds.
Another good analogy of design detection is the Search for Extraterrestrial intelligence. In SETI, researchers search for a complex and specified pattern of electromagnetic radiaton emanating from space. If they find a which matches what they would expect from an intelligent source (such as a series of prime numbers found no where in nature), then they conclude design.
In both the Easter Island and SETI examples, investigators search for complex and specified information to detect design. This is exactly what intelligent design theorists search for to detect design in biology. The analogy thus seems to hold, at least on this minimal level.
It has been suggested that this analogy is inappropriate because it is assumes that similarities between inanimate objects (i.e. the stone faces) and biological ones. The purpose of the analogy is not to say that inanimat objects are the same as animate objects (which would thus create a problem of relevance and dissimilarities), but rather is to merely providing an example of a situation similar in nature to the method of detecting design in biology is the same as detecting design in other fields. In other words, the analogy to the stone faces is supposed to exemplify the same type of design inference process and situation that we use in ID for biological systems.
Finally, one objection put forward by critics of intelligent design is that we understand how humans design human-made objects (i.e. "ordinary design") because we have present-day experience of it. But we don't currently observe non-human designers designing objects in biology, making it "rarefied design," and thus are not justified in inferring design in biology.
This objection fails to recognize that a fundamental tenet of intelligent design theory is that all forms of intelligent designers produce designs with a common property: specified complexity. The proof behind this claim is simple: intelligent agents are capable of employing choice as they design objects. They are able to think with foresight and mentally figure out solutions to complex problems before actually designing. Thus, intelligently designed objects can contain high levels of complexity which conform to a specific pattern of solution to the problem. Thus we need not have direct observational evidence of how any particular intelligent agent acts: since the agent is intelligent, we already know it will produce specified complexity.
Alternatively, a counter-response to this objection is that it does not matter if the designer was "human" or "non-human" to make the general argument as follows:
Thus, even though we have no present-day visual experience of seeing a non-human intelligent agent currently designing things in biology, we can reason as follows:
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