The teleological argument is an argument for God that hinges on the fact that our universe, our planet, and our bodies all appear to have been fine-tuned and designed by a mind greater than ours.
William Paley, an 18th-century Christian apologist, argued this point by using the "Watchmaker Analogy". Modern additions to the analogy detail it as follows:
Imagine you are walking along a beach, and you see a pocket watch that’s been washed up on the shore. A watch only functions if all its parts are present and working; it is irreducibly complex. We observe, even in the smallest of organisms, an inordinate jump in complexity compared to a pocket watch, and while both are irreducibly complex, the organism has the additional, fantastical property of life. If finding a pocket watch upon a shore is indisputably indicative of a designer's presence, how then can something that is also irreducibly complex yet incomparably more intricate be a product of luck?
It would be irrational to say that a working watch evolved out of the ocean, so in both cases, we see that the design must presuppose an intelligent designer.
A counterargument is often made in favour of evolution, stating that what we perceive as intelligent design is simply the product of time and chance. The probability of even a single part of a single cell occurring by chance is estimated at 1/10^164, however. The idea that "randomness" can produce something so intricate can also be comfortably dismissed, as even with our intelligence, we are unable to replicate a similar structure with similar autonomy.
The idea that our lives and surroundings may be the product of an intelligent designer gains immeasurable merit when considering the unlikeliness of our existence.
Because physical constants and planetary positioning dictate the environment for life, any small change would result in the end of our existence—for example, the earth being slightly closer or slightly further from the sun, or gravity's pull being slightly higher or slightly lower—our universe is observed to be ordered. The chance of such things (and much more unmentioned) working in unison to sustain life is unfathomably low, yet an argument may be made that it "could" happen if we observed order coming from disorder.
This is simply not the case, though, and science concurs. The second law of thermodynamics expresses that:
"The total entropy of a system either increases or remains constant in any spontaneous process; it never decreases."
If our universe is really isolated from a guiding force or mind, as some atheists may argue, they cannot account for the structure and regularity of innumerable aspects of our world. Even if chance could give us a rational explanation for why things are the way they are, it would still be an infinitesimal percentage compared to the disproportionately large percentage that chance did not cause everything.