"Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them." -David Hume People often condemn others for using emotion as opposed to reason to make ethical determinations. Sometimes this seems justified: as in a case where someone ignores evidence that suggests their intended actions will have an outcome they'd find undesirable. I will not dispute that reason has an obvious and desirable role in predicting the consequences of our actions. I will demonstrate why emotion is necessary for determining what's desirable in the first place. Imagine a robot placed in a field. It can't help but act logically, so what will it do? What is a reasonable way for the robot to act? It's clear that it will simply do as it's programmed and designed to do. Despite its supposed utter lack of emotion, and the necessity of its actions being dictated by a logical algorithm, it seems incapable of making reasonable choices. As is said by computer programmers: "garbage in, garbage out." Only, in this case, everything seems to be garbage. Like the robot above, men do as dictated by an algorithm. We can't determine what we ultimately should do without simply making assumptions to that effect. We are necessarily guided by unreasonable passions. That men dispute what should be done at all, is evidence that these passions or desires aren't the same for everyone. As such, it is impossible to reasonably determine an objective set of rules that all men should follow. The normative foundation upon which any man determines what should be done does not hold fast for all men. Ethics--all claims as to what anyone should do--are subjective. Nobody has been able to provide an explanation of how we can determine what men should do from only facts about what is true of nature. Lets take a normative statement like "I shouldn't shoot people at random." Any sensible man will agree that such a statement is a good ethical rule. Why? Consider the following: Premise: (A complete description of the universe.) Conclusion: I shouldn't shoot people at random. A description need only consist of descriptive statements. The premise above will not contain the conclusion, as the conclusion is clearly a normative statement. Thus, the statement above isn't tautological: it isn't the opposite of a contradiction. All sound statements in propositional logic are tautologies. Thus, the above statement isn't sound. So why is it true that I shouldn't shoot people at random? Only because I feel that it is. Any attempt to logically justify the statement will come from our desire to do so. It will be born of our passions. So what, then, is the purpose of reason? David Hume figured that out back in the 18th century, and though many metaphysicians have tried, none have been able to convincingly refute him since. When pushed as to why they suspect that he may have been wrong, they simply say that they feel he is. Case in point.