Argument from poor design
Judging the intelligence of a designer who created the natural world would not lead to the conclusion of a perfectly intelligent one. There are a large number of rather stunning defects in nature that a competent designer wouldn't make, the human eye for example certainly relies on the principles of optics, but the ganglion cells are situated such that the nerves and blood which feed the eye are backwards and run across the light sensing cells of the eye, then feed through a hole placed in the center such that humans and other mammals have a blind spot. Appendices serve no purpose and get infected and need to be removed. The human jaw is too small to properly fit wisdom teeth. Embryos sometimes implant themselves outside of the uterus and without abortion would kill the mother. If you were to conclude design, you would need to conclude an idiotic tinkerer rather than a divine perfect creator.
Examples of poor design
The existence of the blind spot in the human eye.
In the African locust, nerve cells start in the abdomen but connect to the wing. This leads to unnecessary use of materials.
An artist's representation of an ectopic pregnancy. Critics cite such common biological occurrences as contradictory to the 'Watchmaker analogy'.The human reproductive system includes the following:
In the human female, a fertilized egg can implant into the fallopian tube, cervix or ovary rather than the uterus causing an ectopic pregnancy.
The existence of a cavity between the ovary and the fallopian tube could indicate a flawed design in the female reproductive system.
Prior to modern surgery, ectopic pregnancy invariably caused the deaths of both mother and baby. Even in modern times, in almost all cases, the pregnancy must be aborted to save the life of the mother.
In the human female, the birth canal passes through the pelvis. The prenatal skull will deform to a surprising extent. However, if the baby’s head is significantly larger than the pelvic opening, the baby cannot be born naturally. Prior to the development of modern surgery (cesarean section), such a complication would lead to the death of the mother, the baby or both. Other birthing complications such as breech birth are worsened by this position of the birth canal.
In the human male, testes develop initially within the abdomen. Later during gestation, they migrate through the abdominal wall into the scrotum. This causes two weak points in the abdominal wall where hernias can later form. Prior to modern surgical techniques, complications from hernias including intestinal blockage, gangrene, etc., usually resulted in death. Other arguments:
Barely used nerves and muscles, such as the plantaris muscle of the foot, that are missing in part of the human population and are routinely harvested as spare parts if needed during operations.
Another example is the muscles that move the ears, which some people can learn to control to a degree, but serve no purpose in any case.
Intricate reproductive devices in orchids, apparently constructed from components commonly having different functions in other flowers.
The use by pandas of their enlarged radial sesamoid bones in a manner similar to how other creatures use thumbs.
The existence of unnecessary wings in flightless birds, e.g. ostriches.
The route of the recurrent laryngeal nerve is such that it travels from the brain to the larynx by looping around the aortic arch.
This same configuration holds true for many animals, in the case of the giraffe this results in about twenty feet of extra nerve.
The prevalence of congenital diseases and genetic disorders such as Huntington's Disease.
The common malformation of the human spinal column, leading to scoliosis, sciatica and congenital misalignment of the vertebrae.
The existence of the pharynx, a passage used for both ingestion and respiration, with the consequent drastic increase in the risk of choking.
The structure of humans' (as well as all mammals') eyes. The retina is 'inside out'. The nerves and blood vessels lie on the surface of the retina instead of behind it as is the case in many invertebrate species. This arrangement forces a number of complex adaptations and gives mammals a blind spot. (See Evolution of the eye).
Six muscles move the eye when three would suffice.
Crowded teeth and poor sinus drainage, as human faces are significantly flatter than those of other primates and humans share the same tooth set. This results in a number of problems, most notably with wisdom teeth.
Almost all animals and plants synthesize their own vitamin C, but humans cannot because the gene for this enzyme is defective (Pseudogene ΨGULO). Lack of vitamin C results in scurvy and eventually death. The gene is also non-functional in other primates and guinea pigs, but is functional in most other higher animals. The enzyme rubisco has been described as a "notoriously inefficient" enzyme, as it is inhibited by oxygen, has a very slow turnover and is not saturated at current levels of carbon dioxide in the atmosphere. The enzyme is inhibited as it unable to distinguish between carbon dioxide and molecular oxygen, with oxygen acting as an competitive enzyme inhibitor. However, rubisco remains the key enzyme in carbon fixation and plants overcome its poor activity by having massive amounts of it inside their cells, making it the most abundant protein on Earth.> The enzyme nitrogenase actually preferentially binds with acetylene over di-nitrogen, despite it being the key enzyme used in nitrogen fixation in many bacteria and archaea.
The breathing reflex is stimulated not directly by the absence of oxygen but rather indirectly by the presence of carbon dioxide. A result is that, at high altitudes, oxygen deprivation can occur in unadapted individuals who do not consciously increase their breathing rate. Oxygen-less asphyxiation in a pure-nitrogen atmosphere has been proposed as a humane method of execution that exploits this oversight.
The unstable hollow bones built for flight in birds like penguins and ostriches, and the Sturdy bones built for non-flight in animals like bats.
Vestigial third molar (Commonly known as wisdom teeth) in humans. Some other primates with differing jaw shapes make use of the third molar.
The vestigial Femur and pelvis in whales, the ancestor of whales lived on land.
We conclude that this argument efficiently proves the non-existence of God. If the natural world is perfect then God exists. If it is imperfect then the the counter-argument- if it's bad design, it's a result of the Fall (Genesis 3:16 has God saying to Eve "I will increase your trouble in pregnancy")- leads to the unfalsifiability of ID theory.