Consider what Ezra Taft Benson says in Fourteen Fundamentals in Following the Prophet...
Therefore, the most important prophet, so far as you and I are concerned, is the one living in our day and age to whom the Lord is currently revealing His will for us. Therefore, the most important reading we can do is any of the words of the prophet contained each week in the Church Section of the Deseret News, and any words of the prophet contained each month in our Church magazines. Our marching orders for each six months are found in the general conference addresses, which are printed in the Ensign magazine.
The living prophet is more vital to us than the standard works.
Basically, what this means is that when it comes to LDS Doctine, there is nothing concrete. Mormons can only find truth in what is being taught currently. So when someone looks at the D&C section 132 and the need for polygamy it is superseded by official decree 1, but only if it is supported by the modern prophet.
Where does this leave truth. Mormons seem to have an evolving truth, where it is one thing one day and another the next. This is so philosophically problematic.
First, Truth is absolute. Relativism is not a philosophical option. There is one truth, and it can not change or waver or it would not be truth. God is Truth. So then, what do we do with statements within Mormonism that contradict each other? I know Mormons will tell me that " That ( referring to the old [ like blood atonement or anything in JOD or HOTC]) was truth for that time, and today we have truth for our time." They do truly believe what it is that was taught by elder Benson.
This, however, creates a serious issue with the concept of evolving truth, that sometimes completely contradicts what was previously true. This rational is at is core covered in logical fallacy. Consider the Law of Non-contradiction. What Mormon's, in fact, have to say about certain teachings of past prophets is that, it was true but it isn't true now. This violates the first law of logic. It also violates the Law of Identity (which says something is what it is), because truth becomes not truth. It is not what it is. Truth by nature must be stagnant, and unchanging. There is no such thing as 'new' Truth. We can discover truth, yes, but it has always been, and it doesn't change, it isn't new.
For Mormons, truth is determined prophet to prophet. They have no stagnant truth. If The prophet were to come out today and say that homosexuality is acceptable, what could Mormons do? They couldn't even go to the Standard Works, because the (believed) truth therein, is less true then the Prophet.
Another area where this comes into play is when people pray about the truthfulness of the BOM. I know that it is frustrating when Mormons pray with people about the BOM and the person says they feel nothing. Or when a person like myself prays about the BOM and it is revealed that it is false. There is a disconnect of truth here. Because the book is either true or not true. And if Mormonism is truly the only way to return to Heavenly Father and wants people to return to him (Eze 18:32) why would He not give such a confirmation? Most Mormons attack the person, for being willing or sincere enough to hear from God. This, however, creates several theological issues, which I will not delve into here, but perhaps will at a later time.
I would also submit that sometimes Mormons just don't know what the truth is on a given subject. I will cite two examples to make this most clear. First, Where did the battles recorded in the BOM take place? There has been no archaeological support for any of these 'Great Battles.' Where is the Truth? Where did they take place? Why Haven't they been found after 60 years? Why are there no maps in the BOM outlining these things? Second, What are the "gold plates" made out of? LDS apologetics cite FARMS will say they are made out of a material called Tumbaga, which is a mix between Gold and Copper. Yet the Church and Mormon documentation disagrees. Mosiah 8:9 says that the plates of the Jaredite's are Pure Gold. And we also see this is a May 15, 1999 article in the LDS Church News - He had also been instructed by an angel, Moroni, who had met with him each year for four years. On his last visit, he was entrusted with plates of solid gold, which he had been translating by the power of the Spirit." So what is the Truth? Are they Gold or Not? How Can any LDS be sure of these things? Personal Testimony, The usual answer leads nowhere, but to more problems within the nature of truth, in that it can be one thing for one person and another for a second. This is Not truth. How Do Mormons Live Without Solid Truth?