halacha: theory and practice
The fundamental difference between the liberal and traditional movements within Judaism is whether halacha is considered binding. There are, of course, subdivisions; the Conservative movement holds that some halacha is different from what the Orthdoox hold. But they both assert that they are halachic movements following a process.
I think one's attitude toward halacha and observance is bound up in what you believe happened at Sinai. If you think that God wrote these specific words, and specified all of the oral law that goes with them, then of course you're going to be very careful in your practice. After all, you believe that God specified all the details, and you don't want to violate God's will.
If you believe that humans wrote the Torah (either wrote it down at Sinai, or wrote it down after the fact), on the other hand, you're assuming imperfect transmission and you're going to be willing to question implementation details. And some people who question might draw non-traditional conclusions, including that this isn't really binding at all.
The body of halacha is huge. Vast. Way beyond the ability of one person to completely internalize. And so, in the halachic movements, the individual does not have the authority to interpret. If you don't know what to do in a situation you ask your rabbi. If he doesn't know he asks his, and so on up the chain until you get an answer. (Or, in the case of the Conservative movement, sometimes two or more responsa supporting different answers, one of which your rabbi will choose.)
In the Reform movement, on the other hand, the individual is invested with the authority and responsibility to make decisions. He's not forbidden from using traditional sources or methods, of course; a Reform Jew could certainly adopt the practice of taking all questions to his rabbi and following the rulings he gets, just like his traditional friends. But he doesn't have to. God gave us brains for a reason, and we're expected to use them. And since (in this view) the Torah isn't the precise word of God, there's more room for questioning and interpretation anyway.
That's the theory. And it all sounds much more tidy than it actually is. In practice, some traditional Jews interpret and some liberal Jews do not follow through on that responsibility.
No traditional Jew takes every question to his rabbi. Congregational rabbis just don't have the time, and I'd bet most Jews don't have the patience. Traditional Jews also interpret halacha. But this isn't "within the rules" of their system, so I wonder how this is justified theologically. (It's easy to justify it practically.)
Traditional Judaism isn't monolithic, either. Practice varies from community to community, rabbi to rabbi, and, as I just described, individual to individual. The common baseline is pretty-well specified; if you drive to your Orthodox shul or are seen eating at McDonald's you'll definitely draw negative attention. But there's a lot more variation in practice than it sometimes seems from the outside.
There's also a custom of bending rules. Traditional Judaism says halacha is binding, but if you can find a loophole, you might be able to do something that would seem to be forbidden. Even, it appears, if that loophole enables you to do something that pretty clearly violates the intent of the halacha.
Talking about loopholes is challenging, though. One person's loophole is another person's legitimate interpretation. I think turning your lights and heat on before Shabbat begins, rather than sitting in the dark and cold, is part of the intent; the Ka'arites called it a loophole. (Actually, they rejected the talmud entirely, but that's the best example I can come up with at the moment.)
But there are practices that seem a little more clearly leaning in the "loophole" direction. On a mailing list a couple days ago, someone talked about the practice of attending (ticketed) football games on Shabbat by sewing your ticket to your coat before Shabbat, walking in, and letting the ticket-taker tear it off. (Of course you walk there and you don't buy any food.) This, to me, is twisting the rules to achieve a result that was pretty likely forbidden in the first place.
Another example: Someone on this same mailing list described how he carries a house key on Shabbat. (He doesn't live in an eiruv.) The problem is that you can't carry things, but you can wear your clothes. So if you can make the key part of your clothing, you're ok. He was arguing, however, that just putting it on a string around your neck doesn't cut it; the key has to be either functional (in an integral way) or beautiful. You could make a beaded necklace with the key as one of the fobs hanging off of it, I guess. What he did was to modify his belt to make the key part of the buckle. He needs the key or his pants fall down. But to me, this is rules-lawyering well beyond where it should have gone. When I am in that situation, I carry the key and make no apologies. I don't pretend to be doing something else.
One more: I was once at a gathering on Shabbat and an auction was to take place. On my way out of the room, I ran into someone who asked why I wasn't staying. When I said Shabbat precluded the auction, I was told not to worry; arrangements had been made so we could bid now and pay after sundown. If the halacha is about making a financial transaction then that's legit (and I very much respect the person who told me this, so that's probably the case), but the whole point of the halacha seems to be about not doing business, and I can't reconcile bidding with "not doing business".
While I can be just as intellectually stimulated as the next guy by the fine distinctions of rules-lawyering, I don't accept these application when determining my observance patterns. Maybe that's connected with how I view revelation; I think God gave us (many) specific directives and commands, but I think we weren't supposed to lose track of the meta-issues. Those meta-issues include skipping the Steelers game, wearing actual clothing but not fake clothing, and not shopping.
I do sometimes attend activities on Shabbat (pre-paid or free) that might be questionable on Shabbat. I carry with or without an eiruv. I even ride in cars and (very rarely) drive, if it's important enough. I do not, however, do any of this with the illusion that my activities are in keeping with the traditional understanding of halacha.
I know that the Torah does not require us to be unnecessarily strict, and that adding to the law is as much an aveira (sin) as subtracting from it is. We're allowed to interpret; we can put our lights on timers rather than sit in the dark, and so on. But when have we crossed the line from reasonable accommodation into rules-lawyering to get the outcome we want? How can we tell? Within halachic movements I would expect this to be an important question, yet I don't think I've ever heard a serious discussion of it.
Personally, I am fairly observant, including some halacha that I'm not really convinced on (but community is important too). In many areas my default approach will be to do, rather than to not do; I don't have to personally research every subject. But even if my practice were 100% in compliance with that of the canonical traditional Jew (should such a person exist), I wouldn't be a traditional Jew and I wouldn't represent myself as one. That's because I followed a very different process to get there. And that process is founded in beliefs about revelation.
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On asking: from watching and talking with ordinary people, mostly. Maybe my sample set is flawed?
On interpretation: that I'm more sure of. I've definitely seen serious Orthodox Jews doing things that are conventionally held to be violations of halacha. (For example, touching people of the opposite sex.) They aren't apostates who don't care, so they must have reasons. Usually it's impolite to ask, however. (I don't want an innocent question to be mistaken for chastisement.) One time I did ask and got the answer that the person disagreed with the conventional understanding.
So I just assumed this sort of thing was normal.
And, do most/all extra stringencies come from rabbinic rulings, or do most people who take them on do so individually? (Glatt kosher, not using a perfectly functional eiruv, shaving the head rather than covering hair as a matter of halacha rather than convenience, two sets of t'fillin (conventional and Rabbenu Tam)...)
I agree that you need to pay attention to both letter and spirit. If forced to choose I would choose spirit, but that's because I believe differently about revelation.
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The question is more, is it a custom or a law? I'm not sure, but "negiah" may well be a custom, and so, while minhagim are supposed to be binding, if someone disagrees, he's not violating anything.
And, yes, it does happen a lot that someone looks at a custom and decides that the conventional understanding of the underlying law is wrong. And he or she can usually find sources to back him or her up. But. They have to be certain that it is only a custom.
From what I can see, most chumrot (stringencies in customs) come from ignorance or zealotry, or even a fear of asking questions. Shaving the head, btw, is not considered halacha anywhere. Even those groups who do so regard it as a binding custom, and understand it's for convenience. Binding customs have the almost the force of law - why I don't eat peas during Pesach, for example - but they are not laws.
Glatt kosher, btw, is not a stringency. Like many halachot, there are three stances possible - leniencies, "stam" and stringencies. For Ashkenazi Jews, there is a leniency that permits non-glatt meat, so for them it's kosher. For S'phardi Jews, no such leniency exists, and they must eat glatt - and Ashkenazi glatt isn't strict enough for them.
And I'm not even sure what the term for using two sets of t'filin is, other than the custom of Lubavitch.
The chances are, if someone asked about something, their rabbi is more likely to give a more lenient answer than they would themselves - example. I accidentally used a plastic meat spatula in a pareve soup made in a dairy pot. If left to my own devices, I'd have discarded the soup and the spatula. But. I called my rabbi. Pot is kosher. Soup is kosher, but should be served in pareve or disposable bowls. Spatula is kosher. (Another rabbi we asked after receiving and acting on this p'sok was a bit stricter, but we were already covered.)
There is also a thing in the more right wing groups to be "frummer than thou" - the more stringencies the better. It's a form of zealotry that makes an already fragmented group even more fragmented, imho.