How are YOU able to determine that MY marginal utility from investing X% of my resources in feeding the homeless, for example, is absolutely LESS than the utility you derive from going to the movies?
It's impossible for YOU to determine that.
We can study the difference. We can measure similar differences over time. But there's no possible way for you to say the utility you derive from a 2 hour movie is more or less than the utility I derive from feeding the homeless for 2 hours.
Further, the motivation as to WHY I would prefer to feed the homeless for those 2 hours is ENTIRELY IRRELEVANT. The ONLY thing that matters, that would make my investment rational or irrational is whether that measurement of utility is POSITIVE OR NEGATIVE.
A primitive illustration is this. While I dont agree with this or do it, let's take someone who does. One could potentially derive POSITIVE utility from smoking a joint. However, if all you do is smoke joints to the extent you lose your job, your home, your girl, all your money and end up in a slammer, somewhere in there the UTILITY of smoking the next joint turned negative.
Whether that first puff was religiously motivated or the last puff from the 100th joint was smoked by an anarcho-capitalist atheist, explain where anything other than UTILITY mattered in the equation of what was rational?
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