It is possible to blend Hume’s is-ought distinction (1) in Ethics with Plato’s justified-true-belief theory of knowledge.
Simply put, whatever sort of beliefs one is talking about, including moral beliefs, they must be ‘both’ justified by reasons (a justified belief OUGHT to be believed) ‘and’ correspondent to reality (a true belief IS true). Just because one has good reasons for one’s belief does not mean it is true. And even if one believes something that is true, one may have horrible reasons for believing it (bit of a tangent: Gettier was wrong in assuming falsehoods count towards justification) (Gettier is answered elsewhere (2)). All of this is true about any belief one holds, moral or otherwise. In order for one’s belief to be knowledge, it must satisfy those two conditions: 1: It must be backed by good reasons (justified/ought). 2: It must correspond to reality (true/is). These conditions are very different from each other. Both are required separately. So, satisfying both conditions is not Hume’s problem—it is when one condition takes the place of the other that one commits Hume’s is-ought fallacy, or its reverse (ought-is).
Hume obviously only drew this distinction when he was discussing moral knowledge, not any other kind of knowledge, and Plato grappled with Euthyphro’s (false) dilemma (it, skeptics, anti-realists and Gettier are all answered elsewhere (2)).
If one understands the blending and one is not a Christian, one may not be comfortable with it because one’s moral theory doesn’t correspond to, or describe, anything in reality, knowing of no always-good person who never has and never will violate one’s moral theory. That discomfort, though understandable, is not a valid reason to reject the is-ought distinction.
If one is a theist who still rejects Hume’s is-ought distinction because one thinks it means the Good cannot correspond to God, then one is misunderstanding what Hume really meant by his distinction, and there is still some work to do in communicating the blending properly. Even educated Christians like Dr. Richard Weikart and Dr. William Lane Craig share such a misunderstanding. Dr. Craig says:
It is unclear here whether or not Dr. Craig thinks the is-ought fallacy is a real fallacy. He seems to when he dismisses the idea that “because God is a certain way we ought to behave in certain ways” (ibid). But he seems not to when he asserts “our moral obligations and prohibitions arise as a result of God’s commands to us” (ibid). It is a tangent, but he also unnecessarily distinguishes between moral obligations and moral values. Anyway, to say “because this is God’s command, we ought to behave according to it” commits the is-ought fallacy.
There has been some discourse with Dr. Craig on this matter via Facebook (5), and via email with Dr. Weikart. Dr. Weikart’s most recent reply just restates his misunderstanding:
Grounded in, yes. Justified by, no.
1. Ichthus77. (2011). Where I am at with Hume’s is-ought distinction. Retrieved from http://ichthus77.blogspot.com/2011/07/where-i-am-at-with-humes-is-ought.html
2. Ichthus77. (2011). Answering Gettier. Retrieved from http://ichthus77.blogspot.com/2011/01/answering-gettier.html
3. Reasonable Faith. (2012). Moral Argument for God. Retrieved from http://www.reasonablefaith.org/moral-argument-for-god#ixzz23vssuCM3
4. Reasonable Faith. (2012). Does Theistic Ethics Derive an “Ought” from an “Is”? Retrieved from http://www.reasonablefaith.org/does-theistic-ethics-derive-an-ought-from-an-is#ixzz23vtucp00
5. Ichthus77. (2012). Is-ought discussion with WLC. Retrieved from http://www.ichthus77.blogspot.com/2012/01/is-ought-discussion-with-wlc.html