The premise that a human being's function is based solely on the optimal use of reason in accordance with the best and most complete virtue has, for good reason, attracted considerable criticism. This is often the case because the function argument in Aristotle’s Nicomachean Ethics is seemingly simple and compact, and therefore lends itself to questionable premises. This essay is concerned with dealing with some of these questionable premises, investigating four specific premises that critics have questioned in relation to Aristotle’s function argument and what it is for a human to function as a human. These are, (1) is Aristotle right in thinking that it necessarily follows, given other things possess a function, that human beings also have a function? (2) Must the function of human beings be unique or distinctive? Why is it that the human function cannot be shared with other living organisms? (3) Why must something’s good reside in the function it performs; and why must it perform its function well, rather than just satisfactorily? (4) Why does performing one’s function well represent a fulfilled or ‘perfect’ life? While there are other questions that can be raised in relation to Aristotle’s function argument in relation to human beings, this essay will deal only with these four main issues. The essay will grapple with each individually, pointing out both their weakness and strengths.