In this section of Nicomachean Ethics, Aristotle seems to offer an argument for the existence of a single "best good," but he is in fact offering a conditional argument for what the highest motivating end would be, if existent. He presents a dichotomy: Either desire is empty and futile, or it is not. Supposing that desire is not futile, he proposes the following:
IF (p): there were a single end motivating all human actions and wishes—an end desirable per se,

THEN (q): such end would obviously be "the best good".

This conditional argument is not offered with any supporting evidence, and it need not be. By itself, the statement is axiomatic: IF there were only one thing we were to choose because of itself, THEN that one thing would be called good. Aristotle proposes only that 'the putative single end which is desirable per se', be renamed, "the best good". Accepting this only obliges us to agree to regard a putative, single, intrinsically desirable end as being "good".

Aristotle’s argument is perfectly acceptable, even without a detailed description of what is "good", because he is proposing only a hypothetical situation. Although it may appear on first reading that Aristotle asserts that there IS a single "best good", he does not. The appearance to the contrary is owed to the subjunctive mood of English, which, while existent, is not readily distinguished from the indicative mood. The inclusion of the words, "Suppose," and "if," are evidence that the passage was written entirely in the subjunctive mood. He leaves open the possibilities that desire is either empty and futile (and that therefore there is NO ultimate end); or that there is more than one intrinsically desirable thing.