Is Rome the answer?

A stock objection to sola Scriptura is that Scripture alone is not enough to furnish answers on important ethical questions. That's why some erstwhile evangelicals convert to Catholicism. That's a standard argument in the repertoire of Catholic apologetics. 

However, it presents an interesting conundrum for Catholicism. For instance, Catholic moral theology regards lying as intrinsically wrong:

http://www.vatican.va/archive/ccc_css/archive/catechism/p3s2c2a8.htm

That might seem to simplify your moral options. If lying is always wrong, then you must take that option off the table. 

By contrast, a Christian ethicist who doesn't think lying is always wrong has the more complicated task of attempting to provide criteria that distinguish licit from illicit lying. So the Catholic position appears to have the benefit of moral clarity. However, the Catholic prohibition is deceptively simple inasmuch as the definition of lying is philosophically intricate and unsettled:

http://plato.stanford.edu/entries/lying-definition/

There's no agreed upon definition of what constitutes a lie. And to my knowledge, the Magisterium has never provided an authoritative definition. Indeed, I doubt it's even possible for the Magisterium to define a lie with sufficient precision to rule out various counterexamples. 

So that generates a Catholic conundrum: Rome says "never tell a lie" without defining its terms. "Don't do it!" but we won't explain what you're not supposed to do. 

What's the point of saying X is forbidden if you don't say what X is? What's forbidden? The prohibited activity is a placeholder waiting for a definition. Is like warning people not to cross an invisible line. "Don't cross that line!" What line? Where's the line? 

The Magisterium issues an authoritative prohibition on lying without issuing an authoritative definition of the prohibited activity. No compact definition will have the specificity required to rule out every conceivable scenario. 

So even if you wish to obey Rome's prohibition, you must figure out for yourself what the prohibition actually means or entails. The definition of lying must be supplied by something outside the Magisterium. By fallible, inconclusive, incomplete philosophical reflection. 

Incidentally, this generates a parallel problem for Scripturalism. According to Scripturalism, the Bible is the only source of knowledge. So how does a Scripturalist define a lie? Invoking the coherence theory of knowledge is inadequate because, even if we grant the coherence theory of knowledge, that's at best, a necessary rather than sufficient condition of a lie. Even if every lie is a contradiction, every contradiction is not a lie. The definition of a lie is more specific than a general theory of knowledge. So the definition of lying is underdetermined by the coherence theory of knowledge.