However, ethical intuitionism is at a minimum a view in moral epistemology according to which some moral truths can be known without inference. That is, the view is at its core a foundationalism about moral beliefs. Of course, such an epistemological view implies that there are moral beliefs with propositional contents; so it implies cognitivism. However, both moral realism and ethical non-naturalism are not essential to the view; most ethical intuitionists simply happen to hold those views as well.
Likewise, sometimes the term "ethical intuitionism" is associated with a pluralistic, deontological position in normative ethics, a position defended by W.D. Ross. However, as is customary in contemporary philosophy, the term "ethical intuitionism" will be used in this article to refer to the general position that there are basic (non-inferential) moral beliefs. Thus, this usage encompasses both empiricist and rationalist accounts of non-inferential moral knowledge. While the empiricist version of ethical intuitionism is standardly called "moral sense theory" (or sometimes "sentimentalism"), there is no standard name for the rationalist version. In this article, the rationalist version of ethical intuitionism will simply be called "rationalist ethical intuitionism".
Generally speaking, rationalist ethical intuitionism models the acquisition of such non-inferential moral knowledge on a priori, non-empirical knowledge, such as knowledge of mathematical truths; whereas moral sense theory models the acquisition of such non-inferential moral knowledge on empirical knowledge, such as knowledge of the colors of objects (see moral sense theory).
Some rationalist ethical intuitionists characterize moral "intuitions" as a species of belief (for example, Audi, 2005, pp. 33-6) that are self-evident in that they are justified simply by virtue of one's understanding of the proposition believed. Others characterize "intuitions" as a distinct kind of mental state, in which something seems to one to be the case (whether one believes it or not) as a result of intellectual reflection. Michael Huemer (2005), for example, defines "intuition" as a sort of seeming:
Reasoning sometimes changes how things seem to us. But there is also a way things seem to us prior to reasoning; otherwise, reasoning could not get started. The way things seem prior to reasoning we may call an 'initial appearance'. An initial, intellectual appearance is an 'intuition'. That is, an intuition that p is a state of its seeming to one that p that is not dependent on inference from other beliefs and that results from thinking about p, as opposed to perceiving, remembering, or introspecting. An ethical intuition is an intuition whose content is an evaluative proposition. (§5.2)
Regardless of one's definition of rational intuition, intuitionists all agree that rational intuitions are not justified by inference from a separate belief.
One way to understand the moral sense is to draw an analogy between it and other kinds of senses. Beauty, for example, is something we see in some faces, artworks and landscapes. We can also hear it in some pieces of music. We clearly do not need an independent aesthetic sense faculty to perceive beauty in the world. Our ordinary five senses are quite enough to observe it, though merely observing something beautiful is not by itself enough to appreciate its beauty. Suppose we give a name to this ability to appreciate the beauty in things we see: one might call it the aesthetic sense. This aesthetic sense does not come automatically to all people with perfect vision and hearing, so it is fair to describe it as something extra, something not wholly reducible to vision and hearing. As the aesthetic sense informs us about what is beautiful, we can analogically understand the moral sense as informing us of what is good. People with a functioning moral sense get a clear impression of wrongness when they see puppies being kicked, for example.
H.A. Prichard gave an early defense of the view in his "Does Moral Philosophy Rest on a Mistake?" (1912), wherein he contended that moral philosophy rested chiefly on the desire to provide arguments starting from non-normative premises for the principles of obligation that we pre-philosophically accept, such as the principle that one ought to keep one's promises or that one ought not to steal. This is a mistake, Prichard argued, both because it is impossible to derive any statement about what one ought to do from statements not concerning obligation (even statements about what is good), and because there is no need to do so since common sense principles of moral obligation are self-evident.
Prichard influenced G.E. Moore, whose Principia Ethica (1903) argued famously that goodness was an indefinable, non-natural property of which we had intuitive awareness. Moore originated the term "the naturalistic fallacy" to refer to the (alleged) error of confusing goodness with some natural property, and he deployed the Open Question Argument to show why this was an error. Unlike Prichard, Moore thought that one could derive principles of obligation from propositions about what is good; Moore believed that what one ought to do is always determined by what will produce the most good.
Ethical intuitionism suffered a dramatic fall from favor by the middle of the century, probably due in part to the influence of logical positivism, in part to the rising popularity of naturalism in philosophy, and in part to philosophical objections based on the phenomenon of widespread moral disagreement.
Some recent work suggests the view may be enjoying a resurgence of interest in academic philosophy. Robert Audi is one of the main supporters of ethical intuitionism in our days. His 2005 book, The Good in the Right, claims to update and strengthen Rossian intuitionism and to develop the epistemology of ethics. Michael Huemer's book Ethical Intuitionism (2005) also provides a recent defense of the view. Furthermore, authors writing on normative ethics often accept methodological intuitionism as they present allegedly obvious or intuitive examples or thought experiments as support for their theories.
Ethical intuitionists could respond by arguing that the moral-aesthetic analogy is merely used to illustrate the fact that not all senses are limited to the five physical senses. However, this doesn't necessarily imply any further connection between the moral and aesthetic senses.
Ethical intuitionists could respond by pointing to numerous long-standing debates in philosophy and science. There's no long disputes about whether some object is green, because that's simply a matter of observation, whereas metaphysics, epistemology, evolution, and quantum mechanics require reason, which is prone to human error. A similar analogy could be made about moral sense, which might give us a clear idea of what "rightness" is, but perhaps doesn't grant us direct knowledge of normative ethics.
A counter-objection is that the very fact we do postulate the independent feature regarding morals is indicative, and the fact we do not feel the need to do this for queasiness is itself an argument for objective morality. Moral sense might inform us of the existence of objective morality, just as eyesight informs us of the existence of colors. However, Occam's Razor wouldn't dismiss the existence of eyesight or colors, but would require the simplest explanation of how they work.