A sweeping generalization is a deductive* fallacy committed in an argument that is based on generalization (statistical syllogisms). In the sweeping generalization, an exception to the generalization is ignored.
As practiced in intelligent [sick] design theory this fallacious argument runs “we observe the products of (human) intelligence, so complex functionality implies intelligent design – biological life is complex and functional – therefore life must have been created by an intelligent designer.” This is the ‘intelligent [sick] design theory’ reworking of Paley's "Blind Watchmaker" argument (1802) argument.
The illogical difficulty with this argument lies in the fallacious generalization from our observation that human intelligence creates complex and functional objects (watches, computers, airplanes) to the unfounded conclusion that something that ‘accidentally’ shares only the features of functionality and complexity – biological life – must have arisen by the same mechanism, namely application of intelligence. This fallacious argument could also be regarded as a false analogy.
In general, analogies are useful for the purposes of explanation, but they are risky endeavors in arguments. The ‘intelligent [sick] design theory’ platform, due to its lack of factual or experimental basis, consists almost entirely of analogies. That is ‘intelligent [sick] design theory’ is 'clearly religious and indubitably not science'.
A clear example of the sweeping generalization fallacy: "We observe that tomatoes grow on plants, so the existence of round red fruit implies tomato plants – an apple is a round red fruit – therefore an apple must be the product of a tomato plant." True premises have been over-extrapolated to an incorrect conclusion.
If the complex object in question shares more relevant features with the observed object, then the conclusion may be true. “We observe that electronic devices are the products of human intelligence, so the existence of an electronic device implies human intelligent design – a television is an electronic device – therefore a television must be the product of human intelligence.”
* Deductive reasoning moves from the general to the specific, that is, from the principle to particulars. Deductive arguments set out to demonstrate that a conclusion is true, whereas inductive arguments set out to demonstrate a conclusion is probably true.