So I’d like to bat a topic around: what is the proper domain of revelation? That is, what is the proper range of subject matter on which prophets can speak with divine authority? Are there subjects where prophets are on more shaky ground to invoke divine inspiration? How are we to tell?
The answer to the domain question will vary depending upon the source. The scriptures teach that virtually every topic is fair game for divine revelation. James teaches that if you lack wisdom, ask of God. In the scriptures, God is the source of all truth and light, and prayer and the Holy Ghost are the proper medium for receiving all truth and light. Moses learns from God astronomy, biology and other sciences. Joseph Smith receives revelation on history of peoples in the Americas. All topics are fair game, with a similar methodology: God bestows answers upon those who inquire diligently of Him.
Science, on the other hand, does not exclude God as a source of truth; it is entirely possible that the Creator told you the secrets of the universe. That said, learning something from God is no different than learning something from your milkman or from your green grocer: it’s not backed by empirical evidence, experiment or process such that would permit repeatable verification. So science does not exclude certain domains a priori from divine inquiry but it does expose all received information to similar processes and testing as a matter of validation.
Further, science has little to say (so far) on more metaphysical matters, such as the redemption of the soul, the eternal nature of human relationships or the efficacy of the atonement. These spiritual affairs do not lend themselves to testing under controlled conditions or the scientific process in general . For some, this means that such spiritual matters are untrustworthy or delusions, but for others it may form a line of demarcation: these are the truly important topics, where science cannot help us, but God can still speak to us. This may lead us to conclude that this is the sole proper domain of revelation, and that when prophets stray from this they are not speaking within their mantle. Such a conclusion is not supported by scripture .
We could consider a spectrum under which to scrutinize pronouncements from the pulpit: the more they deal with spiritual concerns, the more weight they should hold . Conversely, once prophets start talking about the conversion of moon men or white Lamanite remains, the congregation could afford those notions less weight. That is all well and good until the temple recommend interview or other litmus test disregards our personally held sprectra; indeed, given the praxis-oriented perspective of Mormonism, having a personal testimony engineered around a spectrum of ‘weighty’ vs ‘unweighty’ matters is a continual minefield.
Further, consider that you might just be wrong, and that you consider something as non-doctrinal that is, in fact, divine revelation from God; what then? A good example would be the Proclamation of the Family, most of which is innocuous cheering for the Family but some of which deals with thorny concepts like eternal gender or proper roles for men and women; for those that disagree with those issues, it would be tempting to consider the Proclamation (as a whole) as non-scriptural, and those societal aspects as outside the proper domain of revelation. You can disregard a notion more freely if you are convinced it is not of God and is just dicta. Unfortunately for you, it seems that the authors of the Proclamation disagree with your personal interpretation and as a result, the Proclamation can be considered ‘pretty much canon’ for all intents and purposes. See how that works?
Ultimately this is where the exploration of revelation’s domain leads us in Mormonism: it is largely a moot question because of the top-down nature of revealed word. If President Monson would like to give us revelation on tapioca being the most healthy of the puddings and the most beneficial for the treatment of wounds, he can do so and we as lay members are most likely to accept this as truth. Of course, we should trust our leaders and be assured that they know the weight of their mantle – that they would not present something as revealed truth if they had not, in fact, received revelation from God on the topic. This is the basic sort of trust that religion requires in order to work.
If once in a while our leaders speak on topics where they know nothing, what then? History will tend to minimize their words (see above re: Zelph and moon men), but that is cold comfort to those in the congregations at the time. I believe that as the Church has grown in size and in conservativism this has become less of a concern, except when it comes to friction points on hot-button social issues. Thus we may see occasional pronouncements about the nature of human sexuality or taxation or MX missiles, but I think the days of random musings on the unknown are largely over. This is almost bad news: it means that when we are given bold pronouncements that foray beyond ‘purely spiritual’ realms, it will be on matters which will drive people crazy and polarize the congregations. For some, it would almost be preferable to hear about the spaceships of the Lost Tribes and wagonloads of plates.
 Canon and non-canon alike. Indeed part of the canon-forming process may revolve around excision of disproven materials. For purposes of this post I’m using ‘scripture’ in a kitchen sink sense. Moon men and all that.
 For example, if you were unaware of any of those things in the Bible you are probably uneducated.
 It is not clear that JSJ meant for this concept to be available to readers. More likely, it was introduced to justify some of his own words and acts. It’s entirely possible that JSJ would be deeply uncomfortable to hear that readers were reading the Bible and saying, “well in verse X he clearly wasn’t speaking as a prophet.” The issue of infallibility is a tricky one: see here .
 Unless, of course, science leads you to not believe in God in the first place.
 No, Virginia, Moroni 10:3-5 is not scientific.
 But then again, what else would we expect scriptures to say?
 Helpfully (or perhaps spectacularly unhelpfully), JSJ collapsed the spiritual/material distinction when it came to injunctions from the pulpit. “All matters are spiritual” is a good thing to say to your cub troop when they balk at your revelation on using graphite in pinewood derbys.