Analyzing Kant's Function Argument

In the First Section of The Grounding for the Metaphysics of Morals, Kant presents an argument stating that ‘happiness’ is not, in fact, the ultimate and highest end for any being with reason and will, but that the true end for these beings is a will that is good in and of itself, without condition. Kant bases this argument on his belief that the “cultivation of reason”, which is made necessary for the attainment of an unconditionally good will, in many ways, restricts the attainment of ‘happiness’, which, he says, is always conditional (Kant). Seemingly in contrast to Kant’s argument is the argument made by Aristotle in Book I of Nicomachean Ethics. Through what is known as the “Function Argument”, Aristotle develops his claim that the
The second premise is then drawn in a parallel sense from the first such that the function of a human being, who has the ability to reason, is that being’s “preservation, welfare, or in a word its happiness”, meaning that reason is the ability most suited toward achieving the end of ‘happiness’ (Kant). But in comparing reason with instinct, it becomes clear that the parallel that has been drawn is, in fact, false and that reason and will are not the most well suited abilities toward an end of ‘happiness’. This forms the third premise of this argument, that ‘happiness’ is much more easily attained by instinct than it ever could be by reason. These three premises lead to Kant’s conclusion, which states that the function of reason is not achieving ‘happiness’. He then moves forward by supporting this claim by considering those most experienced in the use of reason; they tend to find that they have “only brought more trouble on their heads than they have gained in happiness” and so they “come to envy the more common run of men who are close to the guidance of mere natural instinct” (Kant). But given that humans possess this ability to reason, there must be some end at which it is aimed. Kant believes that the function of reason is to “produce a will which is not merely good as a means to some further end, but is good in itself” (Kant). The good will, while not necessarily the “sole and complete good”, must be the “highest good and the condition of all the rest” (Kant). And if the good will were to be accepted as an unconditioned purpose of reason, it would appear that this same reason would in inhibit ones being able to attain ‘happiness’, which is always