The teleological argument infers the existence of a designer - god, from the apparent design of the universe. For the inference to work the arguments for apparent design must be sound.
To argue for design explicitly is to observe natural organisms closely. Life appears to have a 'telos'. Living creatures are made up of organs that are set to perform a given task. For example, the heart pumps blood around the body and the lungs respires so we can breath. More astonishing however are how small parts of a complex system work so wonderfully and intricately in conjuction with each other to serve a purpose. Suppose the eye. The eye has numerous features that allow sight. If one small part of the stops functioning, it would lead to the over all disfunction of the system. This complexity suggests a plan - a plan that follows a purpose. Thus, there must be an intelligent planner and designer.
Paley develops an analogy to infer the existence of a designer (god) by identifying what it is exactly about human artifacts that leads us to the conclusion that it must have had a designer. When we happen to find a watch on the ground, surely we wouldn't just think that the watch got there by its own or that the watch self-evolved. When we observe the watch closely, we can identify design-like properties. For example, the coordinations of small parts of the machine that allows the hand of the clock to tick in a 360 degree angle and the fact that the watch constitutes the concept of time and numbers, all of which suggest that an intellgient being was behind it. Similarly, Paley argues that the world exhibits similar design-like properties, so we can therefore also infer the existence of a designer on a larger scale.
Hume was unconvinced with Paley's analogy. Hume argues that the analogy given by Paley is un-sound. Throughout the analogy, Paley relies on the idea that human artifacts are not produced by natural means. This is true. We can’?t expect a tornado to produce a lovely home. But in the case of natural objects, they can most certainly be the cause of natural means. Hume says that similar effects (design-like properties) don't necessarily always have similar causes (designer). For example, having a baby can have various causes for example test tubes.
Even if the analogy was sound, and that similar effects do have similar causes, surely that would attribute human like characteristics to God. The quality of the design is said to be to reflect the power and ability of the designer. In the universe, design is not so perfect because we see natural distasters such as volcanoes that cause suffering. Thus hume argued that God is not perfect as we think he is. So the question we can pose supposing that the analogy is sound, is whether benevolent and onmipotent God can be reconciled with suffering.
Nonetheless, given that similar effects don't always have similar causes, what explains the appearance of design in natural objects if they havnt actually been designed? To explain what natural things appear designed, we can appeal to Darwinism. What appears to be designed is just the product of good scientific functioning. Hundreds, if not millions of random alterations take place in the living creature. An organism that coincidentally possesses an advanced trait in the genes, adapt to their hostile environment and stand a greater chance of survival. Other organisms that are not genetically advanced as others will not adapt to their hostile environment and will as a result die. Those who are stronger pass on the trait to their offspring and so on.
To provide a more reasoned and rational argument for the existence of a designer, we need to avoid using the analogy. As hume states the well known fact similar effects don't always have similar causes as illustrated by natural selection.
Cosmologists induce the existence of a designer by questioning Darwinism in terms probability. Natural selection may have explained why natural things appear to be designed but it hasn't explain the laws that conspire evolution to take place. Scientific explanation for natural selections do not explain the laws of nature but presuppose other laws. So what explain laws in themselves. Darwinism says natural selection is the process of randomness, but that seems incoherent with our idea of regularity. The universe is ordered, and consistence hence why we know things of it. The idea of life having evolved randomly is devoid of regularity. It is extremely improbable that life had evolved randomly on one of billion stars in a universe where laws had to be so precise and so accurate to allow its own existence to occur. Therefore, it is more probably that a supreme designer indirectly caused life to evolve on planet earth by means of deliberate natural selections as to inspire laws to self-govern in such a way to allow everything to be the way it is than belief it was wall random.
Critics of the cosmological argument suggest if we are trying to give explanations of everything, what explains God? That is something that we don't know. So there's a limit of explanation. So the teleological argument has a dead end. Cosmologists counter argue by suggesting that the probabilty of God being the only explanation increases as more scientific laws are being discovered that we can't explain scientfically. So we find ourselves appealing to God more as we discover more laws that science on its own cannot explain. So the teleological argument is an quantitive ongoing development.