Do we value keeping fish alive more than keeping old people alive via power-hungry air conditioning?
There are millions of similar conundrums, none of which are solvable via appeal to objective truths.
Put another way, you can only have a logical argument with someone who already agrees with your beliefs.
Technically true, but that's an easy (and all too frequently used) way to dismiss other people's arguments and keep believing whatever you want.
It's very unlikely that any real-world policy question is a question about your fundamental value. You most likely don't value marijuana intrinsically. The future of a river dam is probably not the real center of your morality. Rational reasoning is useful to illuminate connection between the subject of an argument and your moral values. A connection that usually is pretty complex, and allows for honest and patient enough people to reach the same conclusion given the same knowledge about the problem.
Consider the towns and cities by bodies of water that have either dried up or been polluted. It's financially ruinous because it either/or kills tourism and/or fishing industry; the salty dust left behind by water causes horrible health problems; there's natural brain/brawn drain that leaves only the older and sickly behind to either fend for themselves or be on gov pay check. Just off the top of my head. I can easily think up a lot more effed up scenarios that have already happened in the world (see Aral Sea).
But along the way to the unfortunate end you describe above, many good things came of the dam. Any calculus needs to evaluate the tradeoffs. "Were they worth it?" doesn't lend itself to objective-factual analysis, because "worth" is all about values.
So are engineering problems.
Those tradeoffs are "hard to decide via objective truths" only because they haven't been decomposed enough. You have to dig deeper into them, split them apart, ask why do we care about something, and then ask again and again. It might get hard to keep track of the expanding tradeoff space, which is probably why people shy away from digging into things too deep. But if you do the work, you may find out that there is a way to solve the original problem in a way that satisfies everyone's low-level values.
Reminds me of essays of Bret Victor, who occasionally seems to be making a related point - a lot of policy debates quickly devolve into appeals to emotions, because we lack tools to effortlessly decompose and analyze actual problems. See e.g. http://worrydream.com/ClimateChange/#media for a good example.
No, it's not the only way. It's the easy way.
The hard way is to consider other consequences of both choices, including long-term effects, including changes that aren't 0%-100%, but slightly change probability of other events.
Hurting the ecosystem might increase the probability of flooding the area, which will hurt some of the people that you are trying to comfort by giving them cheaper energy. So, maybe it's objectively irrational to build the dam, no matter whether you value people or fish more?
Maybe the population in the area is decreasing anyway and cheap power won't help? We can predict such things pretty reliably. So why build a huge dam and hurt environment if in 40 years it will be useless?
It's impossible to know for sure, but it's possible to estimate. Abandoning the rationality altogether just because it's not immediately obvious what to do is very irresponsible.
I know it was just an example, but IMHO it's a representative example. Many of the issues in the center of the public debate now are objectively scientifically solved, or so close to be solved, that it doesn't matter what the exact details are.
Yet people continue to argue about them because of tribal thinking, religion, corporate interests, etc. See global warming, equality for sexual orientations, war on drugs, public vs private healthcare, etc.
OC proposed that human element is inherently unpredictable, so rationality isn't the solution. Yet - thanks to the law of big numbers - big groups of people are more predictable than small groups of people or individual people. So we may not know who will vote X or Y, but we can predict how many people will vote X and Y with reasonably small error. We have big data, that can predict you're pregnant before you know you're pregnant, just basing on you browser history FFS.
Seriously, a better way would be to start doing 5-whys on this, instead of going for "non-objective things". A quick attempt will tell you that we care for both ecosystem and power, but depending on the local context, we might care more about one or the other. Maybe we can import power from elsewhere, or put a dam in a place where it won't disturb ecosystem so much. Maybe the ecosystem can handle the dam, or really the power is desperately needed.
You can quantify this all if you bother. Problem is, most people don't bother, they very quickly stop thinking about it, and start going by their gut.
(I'd say, they stop behaving like adults, but since a lot of adults have this problem, I guess we need a different phrase.)
Caring more about one or the other is exactly the issue. The reasoning about the degree of which to care about either at all is the issue that cannot be resolved objectively.
Putting a dam in place that won't impact the ecosystem so much--sure, we can minimize the tradeoffs, but the tradeoffs remain, and all resolve to what we value vis a vis what it costs. Which is not objective.
Think of it as akin to mathematics. Mathematics starts with axioms, which can be completely arbitrary, and then derives an entire universe from them by following only objective, formal reasoning. Similarly here, you have some personal values that are (to some extent) arbitrary, but accepting those, you get to use objective reasoning to devise a set of tradeoffs that satisfies those values the best.
Also sometimes different people with different values aren't really in conflict, they just don't have enough knowledge to realize their values aren't being violated. Fix that, and you can get agreement.
Why are those non-objective?
There is no objective answer to that question.
Prove it.
Wrong.
Both the fish and the electricity need to be considered in a long term utilitarian framework.
The only reason you seem to have two conflicting objective truths is because your time window is too short.
Says who?
You have an infinite regress problem here. You eventually are going to have to fall back on non objective feelings about how the world should be to justify your actions.
Depending on how you define "utility" - this is either a meaningless tautology that doesn't say anything at all, or a subjective belief that can be easily challenged by equally subjective alternatives.