It's tiresome and it doesn't add up but it's one of the most popular arguments for God's existence, "the argument by design." Jehovah's Witnesses and other believers point out a wonder of nature like the eye and compare it to a 'lesser' mechanical wonder of human design like a camera. The supposed conclusion is that this natural wonder is evidence of creation by a powerful designer; God.
When you don't examine the argument too closely it seems persuasive and we have to admit it's been powerful enough to fool intelligent people for a long time, but it suffers from some serious flaws.
Now, I'm going to assume that most here are familiar with William Paleys "watchmaker analogy". Over 200 years ago Paley reasoned like this;
"In crossing a heath, suppose I pitched my foot against a stone, and were asked how the stone came to be there; I might possibly answer, that, for anything I knew to the contrary, it had lain there forever: nor would it perhaps be very easy to show the absurdity of this answer. But suppose I had found a watch upon the ground, and it should be inquired how the watch happened to be in that place; I should hardly think of the answer I had before given, that for anything I knew, the watch might have always been there."
Paleys conclusion was that;
"Every indication of contrivance, every manifestation of design, which existed in the watch, exists in the works of nature; with the difference, on the side of nature, of being greater or more, and that in a degree which exceeds all computation."
This is, perhaps, the most famous and well known 'argument by design' although it isn't much of an argument at all, it's more of an analogy really. Now consider this, the 'watchmaker analogy' distinguishes between natural objects and man-made objects. The argument is that if you find a watch on a beach you would immediately recognize that it stood out from the surroundings pebbles, driftwood seagulls and shellfish - it is different. So, you'd conclude it was a human artifact.
BUT.
At this point, the 'argument by design' suddenly requires you to believe that everything in nature shows evidence of a designer!
The analogy contradicts itself at the most basic level. The differences previously observed between natural objects like the pebbles and the human artifact like the watch is what originally made the observer decide that the watch is designed. Those differences are rendered meaningless if we must conclude that the natural objects are designed as well.
Think about it. Why, when you see a watch, do you conclude it is an artifact?
It is its simplicity. Oh I know the believers here will jump all over me and say "No! The watch is complex, that's why it bears evidence of design!" WRONG! Compared to any creature on that beach, the watch is simple. Human artifacts use simple geometrical forms like straight lines and right angles that are actually quite rare in nature.
The argument from design is also dishonestly selective. If, as many theists would argue, the complexity of the universe is evidence of design and that this proves the existence of an even more complex Designer/Creator/God why is that entity’s complexity not evidence of an even more complex deity who, in turn, will also have to be the product of an even more powerful SuperGod? And on, and on, and on . . . Ridiculous, and the arguments given against this are weak at best.
So, just to recap, how does a person distinguish between artifacts and natural objects? Human experience and simplicity of form tell us we are seeing a watch. Conversely, we recognize pebbles, grass and crabs as natural objects. Ok? So why, dear believer do you then turn around and say that these natural objects are actually designed as well! What you are really saying is that there are no natural objects at all only artifacts created by some designer!
The whole 'watchmaker analogy' or argument by design is ultimately self-defeating.
And don't forget evolutionary theory - an elegantly simple explanation of the complexity we see around us. Perfectly understood natural processes will produce beautiful objects of naturally designed complexity.