After the preliminaries in Nicomachean Ethics I.1-5, Aristotle considers a rather abstract question: is there some universal good or "Form of the Good" that is common to all good things? Aristotle's friends in Plato's Academy thought so, but in pursuit of truth Aristotle finds that he must disagree, for several reasons.
First, because "good is meant in as many ways as being is", we can't provide a unified account of the good that explains the goodness of entities that are inherently valuable (e.g., god), good qualities (e.g., the excellences of character), good amounts (e.g., the right level of wealth), good relations (e.g., the usefulness of a tool in relation to a given task), good occasions (e.g., the opportune moment to praise someone), good places (e.g., a dwelling), etc.
Second, if there were one Form of the Good then one kind of knowledge or understanding would encompass them all. However, in fact, the opportune moment (say) in war is studied by the craft of generalship, whereas in health it is studied by the craft of medicine.
Third, it's unclear how knowledge of the universal good (even if we could even acquire it) would actually help weavers or carpenters or doctors to better practice their craft; the gulf between the universal and the particular is too wide: a doctor treats human beings or a particular person, not "the good" in general.
In short, the universal good is not the kind of thing that a human being could know or do or have; what we are seeking is the highest good achievable through human action, not an abstraction beyond our ken. Although Aristotle has his unworldly moments, as we'll see, in general his inquiries stay grounded in human life and human experience. Good thing, too, because we're about to move on (in I.7) to a rather knotty line of reasoning about the characteristic activity or "task" of human beings...
(Cross-posted at philosopher.coach.)
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