For some time philosophers have thought of epistemology and metaphysics as different branches of philosophy, investigating, respectively, what can be known and the basic properties and nature of what there is. It is hard, though, to see any genuine boundary here. The issues irresistibly overlap. Certainly in Plato there is no such divide. His views about what there is are largely controlled by ideas about how knowledge can be accounted for, and his thinking about what knowledge is takes its character from convictions about what there is that is knowable. As a result his doctrines have a different shape from characteristically modern ones.
Some earlier Platonic writings do have a somewhat modern look. Socrates was notorious for having questioned whether he knew much of anything, and for making people hesitant about their opinions (Meno 80c, 86b-c). c). Plato exploits this side of Socratic thought. The namesake of the Euthyphro judges that an action of his is pious. Socrates wonders whether Euthyphro ought to be confident about that judgment, and tries to make him less so. Elsewhere Socrates raises questions concerning his own judgments about which things are beautiful (H. Ma. 286c). Such questions seem to suggest a general policy of doubting, reminiscent to us of Descartes or of the various programs of ancient skepticism. In Socrates' efforts to overcome ignorance (Meno 86b-c) we might see a project of justifying beliefs like that of typical contemporary epistemologists.
Sidgwick believed that there are three main differences between Greek ethics and modern ethics. First, he said that in modern ethics the “imperative” or “jural” or “quasi-jural” notions of obligation, duty, and right are central and the focus is on the question, “What is duty and what is its ground?” In ancient ethics, he said, this question is not asked. Instead, it is asked, “Which of the objects that men think good is truly good or the highest good?” And the “attractive” notion of good is central (ME, 106).
Sidgwick's second difference is this: “It was assumed on all sides [by Greek writers on ethics] that a rational individual would make the pursuit of his own good his supreme aim” (ME, 91–2). The modern view, however, can regard it as rational to take as an ultimate aim something different from and even possibly incompatible with one's own good, namely, right or duty or (in one sense) virtue.
Sidgwick thought that this feature of the ancient view, its acceptance of what I shall call “rational egoism,” is compatible with saying that ancient accounts of conduct are “moralities.” On some taxonomies a view does not count as a morality if it either says or is supported by a rationale that says that one's own good is one's ultimate rational end. A taxonomy of this kind holds that such a view is too much like a kind of egoism to be called a morality.
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