preceding discussion about the ontological categories of values and value-bearers assumes some form of realism: that there actually are valuable things. But the difficulties in reaching expert consensus in value-related fields like ethics, aesthetics or politics and considerations from naturalism have led various philosophers to doubt this assumption. The ensuing dispute between cognitivists and non-cognitivists is usually held on the level of value-statements or value-attitudes, either concerning all values or specifically concerning ethical values. Cognitivists assert that value-statements are truth-apt, i.e. are either true or false, which is denied by non-cognitivists. Most cognitivists are realists about values: they believe that values are part of reality. Error theory, as originally articulated by J. L. Mackie, is an exception. Error theorists hold that all value-statements are false, and thereby truth-apt, because the world lacks value-features that would be needed to make them true. Non-cognitivists, on the other hand, go one step further by denying that value-statements are truth-apt. This position involves the difficulty of explaining how value-statements can be meaningful despite lacking a truth value. This challenge can be met in different ways. Emotivists, following A. J. Ayer, state that value-statements only express the emotions of the speaker and are intended to influence the actions of the listener. Prescriptivism, as developed by R. M. Hare, interprets value-statements as imperatives or commands. Simon Blackburn's quasi-realism states that value statements project emotional attitudes as though they were real properties. Monism and pluralism Substantive theories of value try to determine which entities have intrinsic value. A traditional dispute in this field is between monist and pluralist theories. According to Chris Heathwood, monism and pluralism can be distinguished according to an evaluation of what is good in people and the concept of "value simpliciter" in terms of intrinsic value. Monist theories hold that there is only one type of intrinsic value. The paradigm example of monist theories is hedonism, the thesis that only pleasure has intrinsic value. Pluralist theories, on the other hand, contend that there are various different types of intrinsic value. They maintain that these types of intrinsic values cannot be reduced to a single feature of an act or entity. W. D. Ross, for example, holds that pleasure is only one type of intrinsic value besides other types, like knowledge. It is important to keep in mind that this disagreement only concerns intrinsic value, not value at large. So hedonists may be happy