Aquinas’s argument of motion has three clear premises and two conclusions. Premise one says there are moving objects in the universe. The second premise mentions, every moving object is set in motion by something else. Premise three stipulates that an infinite regressive chain of motion cannot exist. It leads to conclusion one, therefore, there is a first mover. Conclusion two states that this first mover is God. Aquinas’s premises and the first conclusion are logically valid. The first conclusion logically follows from the three premises or in other words, if the three premises are true the first conclusion is true. Up to that point the argument follows a logically valid structure and premises leads to the conclusion.
However, the problem in Aquinas’s argument lies in the second conclusion. Aquinas jumps from the first conclusion to the second conclusion without any logical relation. Aquinas just assumes that the first mover is God. However, the premises of the argument have only established the presence of a prime mover, but not the presence of God. This prime mover can very well be a physical force or some other type of energy, something on the lines of Big Bang. If the Big bang theory is right, then Big Bang is the prime mover that has set other objects into motion and caused the chain of motion to begin. A prime mover can exist without appealing to the idea of a God. So, the argument is a logical proof for only a prime mover. All the premises of the argument and the first conclusion can be true, but the second conclusion could still be false. Hence, the argument is valid only up to the first conclusion. Once the second conclusion is added the argument becomes invalid as the second conclusion does not follow from the premises.
The second cosmological argument is not much different in structure from the first one. The second argument is the argument of the first cause and it runs as follow. It is possible to perceive a series of efficient causes in the world. But, no object exists prior to itself. So, no object is the efficient cause of itself. An effect or result cannot exist if an efficient cause does not exist. However, the series of efficient causes cannot regress backward infinitely as it would lead to the conclusion that nothing exist in the present. Therefore, there has to be a first efficient cause and that first efficient cause is God. This argument has the same logical form as the first argument and it runs into the similar type of logical objections as the first argument. In addition, the second argument also runs into some informal fallacies as well. As Russell points out, we cannot ask about the cause of a thing that we cannot experience, something like the universe. The universe is “just there, and that's all”
"Cosmological Argument." 13 July 2004. Stanford Encyclopedia of Philosophy. Web. 28 Nov 2014.
Foutz, Scott David. "An Examination of Thomas Aquinas' Cosmological Arguments as found in the Five Ways." n.d. Quodlibet Journal. Web. 28 Nov 2014.