The Fifth Way: Argument from Design - http://web.mnstate.edu/gracyk/courses/web%20publishing/aquinasfiveways_argumentanalysis.htm
1. We see that natural bodies work toward some goal, and do not do so by chance.
2. Most natural things lack knowledge.
3. But as an arrow reaches its target because it is directed by an archer, what lacks intelligence achieves goals by being directed by something intelligence.
4. Therefore, some intelligent being exists by whom all natural things are directed to their end; and this being we call God.
While I am a theist, there exist some gaps in Aquinas’ Arguments for God. These gaps are apparent in his fifth and final argument, which is problematic because his it is unable to tie the five arguments together to create a cohesive argument for God.

First and foremost, premise two is incorrect because Aquinas is contradicting his second argument: Argument for Efficient causes. Premise one states “We perceive a series of efficient causes of things in the world” (from the same link as above). Perceiving events is a type of knowledge. If we, as humans and “natural beings,” did not perceive events there would be an absence in our understanding, otherwise known as a lack of knowledge. Since this knowledge is fundamental to understanding causation, we must inherently have this knowledge in our natural state. So, premise two, most natural things lack knowledge, is false. (Or this premise could stand, but the rest of the argument would be false because the two premises are contradictory).

Next, premise three has some issues. If what lacks intelligence achieves goals by being directed by something of intelligence, and this intelligent being is God and the unintelligent are humans, then one would assume that the goals hoping to be achieved would be virtuous. However, the history of humanity shows that there are many instances of humans pursuing non-virtuous goals. For example, the residents of Sodom and Gomorrah chose to pursue pleasures of the flesh, even when Lot warned them of God’s wrath towards the sinful, over the possibility of salvation. Two cities were smit by God because they chose goals that did not align with God’s virtues. Rather than “working towards some goal” and “not [doing] so by chance,” they actively work against what Aquinas claims they are naturally predisposed to. This raises the questions of did God lead them to ruin, or did God let them exercise their free will, thus acting contradictory to premise three. Either option is problematic to the existence of a benevolent, Creator God.