The philosopher David Hume said, "when you pronounce any action or character to be vicious, you mean nothing, but that from the constitution of your nature you have a feeling or sentiment of blame from the contemplation of it". What did he mean by this?
To understand Hume's assertion, we need to back up a bit and discuss the ways in which statements can be true. Suppose we have a statement like "Grass is green". Under what conditions is that statement true? The answer is deceptively obvious: "Grass is green" is true iff grass is green. We call the part after the 'iff' the statement's truth conditions.
There are different kinds of truth conditions. Let us say that truth conditions are objective just in case they are publicly available and publicly debatable. The truth conditions on the statement "Grass is green" are objective because we all have access to grass and most of us can perceive the color green. Grass, in short, is something we can look at and talk about.
Of course, some things we can't look at. Suppose I say, "2+2=4". This is objectively true, of course, but it's not objectively true in the same sense in which "grass is green" is true. So we draw a distinction between kinds of objective truth conditions. We say that the truth conditions on statements like "grass is green" are objective a posteriori and the truth conditions on statements like "2+2=4" are objective a priori.
Truth conditions which are objective a posteriori require some experience of the world in order to verify their truth: these are sometimes called 'truths of fact'. Truth conditions which are objective a priori do not require any experience of the world in order to verify their truth: these are sometimes called 'truths of reason'. The statements of logic and mathematics have a priori objective truth conditions, while the statements of biology and chemistry have a posteriori objective truth conditions.
Other truth conditions are neither publicly available nor publicly debatable. Suppose I say, "I have a toothache". Presumably, this statement is either true or false. But it would be absurd for you to say, "Oh no you don't!" Only I have access to my inner, subjective experience. So the truth conditions on statements like "I have a toothache" or "I like chocolate ice-cream" are subjective.
In the above passage, Hume is simply claiming that ethical statements do not have objective truth conditions. What leads Hume to make his famous statement is something very much like the following argument:
|1||If ethical statements have objective truth conditions, then ethics is like science.|
|2||There is widespread agreement on fundamental matters in science and there is no widespread agreement on fundamental matters in ethics.|
|3||There are established methods for resolving disputes in science and there are no established methods for resolving disputes in ethics.|
|4||It is possible to resolve disagreements rationally in science and it is impossible to resolve disagreements rationally in ethics.|
|5||If (2 , 3, and 4), then ethics is not like science.|
|∴||6||Ethics is not like science.||2,3,4&5|
|∴||7||Ethical statements do not have objective truth conditions.||1&6|
Hume's argument, however, appears to miss an important point. There are two ways in which ethical statements can have objective truth conditions. In particular,
How Ethical Statements Can Have Objective Truth Conditions
A. Ethical statements could have truth conditions which are objective a priori.
Ethics could be objective in the sense that moral problems can be solved by rational methods. Moral judgments are rationally justified. In this sense, ethics is objective in much the same way that mathematics is objective. Moral facts are facts of reason. This view is sometimes called 'Moral Rationalism'.
B. Ethical statements could have truth conditions which are objective a posteriori.
Ethics could be objective in the sense that moral predicates like 'right' and 'wrong' refer to real properties of objects in the world in just the same way that predicates like 'round' and 'red' refer to real properties of objects in the world. Moral facts are not facts of reason. Rather, moral facts are just like any other facts about the world. This view is called 'Moral Realism'. According Moral Realism, ethics is objective in much the same way that we consider the sciences-physics, chemistry, biology, etc.- to be objective.
It is clear from his argument that Hume ignores (A). For Hume, the claim that ethical statements have objective truth conditions is equivalent to Moral Realism. So premise (1) of Hume's argument is false. Ethics need not be like science for ethical statements to have objective truth conditions. It could be like mathematics or logic.
In response, Hume argues that reason alone cannot determine morality, since "[m]orals excite passions, and produce or prevent actions. Reason of itself is utterly impotent in this particular." That is, Hume thinks morality plays a role in our lives in determining action which pure reason, exemplified in subjects like mathematics or logic, does not. Morality motivates, whereas the conclusions of mere reason do not. Hence morality cannot also be a matter of reason. It is on this point that his successor Immanuel Kant will press to construct his theory of morality.