I think God revealed a legal code to us on Mount Sinai, but others disagree. So let's leave aside that question for the moment and discuss what happened next. Irrespective of the origins of the legal code, men have been using their limited, fallible, and subjective intellects to interpret it for well over 2000 years. At one point, they read the law in a way that permitted slavery and plural marriages; later they imposed new imperatives on the law and decided that neither practice could continue. At one point, they read the law in a way that outlawed interest and required us to forgive debts every 7 years; later they imposed other imperatives on the law and now we charge interest and debts are not remitted during shmitta.
What happened? The convenient explanation is that morality changed so the law had to change with it. Unfortunately, the word "moral" isn't a very good one. When we say that "slavery is morally wrong" do we mean that God disapproves of it or that we, the citizens of this time and place, are against it. Do we mean that slavery is always wrong, or that its only wrong when people say that its wrong?
My proposed solution is to abandon the use of the word "moral" and instead speak in terms of facts.
Its a fact that society works better - people are happier, healthier and wealthier - after slavery is outlawed. At first, human beings did not understand this, just as they didn't understand that drinking sewage water was a health hazard. But in the fullness of time, men came to realize that society could reasonably expect to enjoy better outcomes if slavery was outlawed.
We can say the same about the economic changes our Sages made. In the fullness of time, after enough data had been collected, they came to realize that society would function more successfully if debts could be collected after shmitta and if Jews could charge each other interest. So the changes had to be made. Not because "morality shifted" but because we acquired new facts and a new understanding about what makes a society more successful.
Doesn't God knows what makes a society most successful? Yes, he does. But we have to figure it out on our own. We have to stumble and lurch towards the truth. And during that process of discovery, we tend to read things in light of our existing prejudices. Once, it seemed crystal-clear that the book of Genesis described a recent creation, and a geocentric universe. Now, we know that neither of those proposals are true and we read and interpret the creation story in light of those new, correct facts. Our morality didn't change; the correct facts merely became known to us making the old readings untenable.
We can approach the slavery and economic passages the same way. Once it seemed crystal clear that the Torah wanted society to be organized around kings, slavery and debt remittance every seven years. But now that the facts tell us that people are much better off when those policies are not followed, we have found new ways to interpret those passages. The original error was not God's, but ours. We are the ones who misread the book. We are the ones who interpreted the laws in light of our own incomplete knowledge. Now that we know more, and our facts are better, the passages must be re-examined in light, not of new morality but new facts.
Whether our modern sages choose to lead - as in the case of pruzbal or heter ishka - or follow -as in the case of the Internet which was verboten until we all realized that internet usage was essential to living in the 21st century - this process of reinterpretation in light of new facts continues.
ILLUSTRATION - The Pruzbul update
1 - Pruzbul is not in the torah
2 - I t was established at the end of the bayis sheni era
3- It was established because the sages could see that the original system was flawed. People were not making loans immidiately before the shmita year, which was detrimental to the health and well being of the poor and to general economic growth
4 - So, in light of those new facts, the system was changed. Gods original idea - or more correctly our old understanding of God's original idea - was overruled in light of the new facts.
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