Utilitarianism: An action is right if and only if it produces the greatest happiness. Thus every action is only moraly correct if it results in the largest amount of pleasure, or happiness, and lack of pain, unhappiness.
WHen i say that there are too many variables, i mean practically, there is absolutley no way of knowing what the exact amount of happiness or pain is produced in any situation. If i take someone's pen, i may be a lot happier, because now i have a pen. but what if that person was going to take a test, and now they have no pen? (assuming they fail this test, or do worse because they dont have a pen at first) Their unhappiness far outways my happiness, thus taking the pen is not morally correct. what if, however, the roles are reversed and i have a test and someone else has a pen? The hapiness they have from that pen is less than the happiness generated if I took that pen and was able to do well on my test. Therefore, the moraly correct action would be to steal.
Again, my problem is that there is no way to measure the happiness generated: what if that pen was a family treasure and i stole it not knowing that? the unhappiness of [the person who owns this pen] would then be much greater. but greater than my unhappiness at failing a test? what if i cnat graduate now? or get into a good school? What if the whole test was canceld and it didnt matter anyway? all these variables must be considered by a utilitarian. And as you can probably see, I dont think that's practical at all.