Bernard Williams, answered from the texts and cited to the page.
The distinction, put simply, is this: thick ethical concepts — cruel, courageous, treacherous, generous, chaste — do two things at once that thin ethical concepts cannot. They describe and they evaluate, and the two functions are not separable components that analysis can pull apart.1 A thin concept — good, right, wrong, ought — floats free of any particular descriptive grip on the world.
It delivers a verdict without telling you what it saw. A thick concept tells you what it saw, and the seeing is already the verdict. The contrast matters practically. If I call you unfaithful, I censure you; but I also commit myself to a claim that I must retract if it turns out you went back on no relevant agreement.2 The factual and evaluative aspects are fused, not layered.
That is what makes thick concepts genuinely reportorial in a way thin concepts are not. Now, one consequence of this is that thick concepts can sustain something like knowledge — local, practice-embedded knowledge of how to go on, how to apply the concept, how to correct a misapplication — in a way thin concepts cannot.3 The advisory model of ethical knowledge, as I put it in "Making Sense of Humanity," can work at the level of thick concepts: a reflective advisor and advisee can arrive together at the conclusion that vanity is a failing, conceit a vice, humility not a virtue.4
But it cannot deliver knowledge at the level of choosing between one set of thick concepts and another, or between thick concepts and the thin replacements that contractualism and utilitarianism characteristically install. At that level, what we have is confidence, not knowledge grounded in something broader.5 The political dimension of the distinction is also worth pressing.
Modern pluralistic societies, and the conception of public legitimacy they carry, tend to demand justifications that are procedural, appealing to welfare or consensus — justifications that deploy thin concepts precisely because they are less ethically distinctive and less committal.6 There is a political argument for this, but the argument has a cost: the reduction of ethical thought to thin concepts strips out the substance that thick concepts provide to a genuine ethical life.7
The Hegelian criticism of modern ethical reflection — that it is too abstract and theoretical to provide any substance — lands here, and it lands because the thinning of concepts is itself a kind of impoverishment, not a refinement.8 One further complication, which I pressed in "The Sense of the Past": even if thick concepts can sustain local knowledge, it does not follow that every thick concept picks up on some aspect of the world that any attentive observer must acknowledge.9
The thick concepts of different societies are often not combinable; we have no idea how they might all be mapped onto a single grid. Aristotle held the stronger position — that some definitive set of thick concepts represented the best achievable human understanding — but that confidence is not available to us.10 What we can say is that where a community uses a thick concept well, something is being tracked; what we cannot say is that the tracking generalizes across all thick concepts of all communities into a unified moral reality.
By a thick ethical concept, Williams means a concept which (unlike a scientific concept) has an evaluative aspect—but which also (unlike a 'thin' ethical concept such as that of wrongdoing, which in Williams's view cannot serve, in any straightforward way, to 'report' anything) has a factual aspect.Philosophy as a Humanistic Discipline, p. xvi
If I accuse you of being unfaithful, I thereby censure you; but I also say something that I am obliged to retract if it turns out that you have not in fact gone back on any relevant agreement.Philosophy as a Humanistic Discipline, p. xvi
People sometimes converge in their ethical beliefs, and those beliefs sometimes constitute knowledge. This can happen precisely when the beliefs in question involve a thick ethical concept.Philosophy as a Humanistic Discipline, p. xvi
One can conceive of a reflective advisor and a reflective advisee arriving at some particular conclusions of that order: to the effect, say, that vanity is a failing, conceit is a vice, and humility is not a virtue.Making Sense of Humanity, p. 208
we shall have to accept the strongly anti-foundationalist consequence that ethical knowledge may rest simply on confidence, and not on a broader knowledge.Making Sense of Humanity, p. 208
their conception of public legitimacy is one that encourages institutions to behave as though society were pluralistic, and to adopt styles of justification that are more procedural, or appeal to notions of welfare or consensus that are less committal and less ethically distinctive than 'thick' concepts.In the Beginning Was the Deed, pp. 48–49
it is precisely the use of 'thick' ethical concepts, among other things, that contributes to a more substantive type of personal ethical experience than theory is likely to produce.In the Beginning Was the Deed, p. 48
The familiar (originally Hegelian) criticism of modern ethical reflection that it is too abstract and theoretical to provide any substance to ethical life is well taken.In the Beginning Was the Deed, p. 48
It will, further, have to be true that people who do not use that concept at all are missing something—that the concept 'picks up on' an aspect of the world that an attentive and interested observer should acknowledge. But this further condition does not follow from what is indisputably true about thick concepts.The Sense of the Past, p. 196
Aristotle did hold a quite strong position about such concepts: that a definitive set of them represented the best achievable human understanding.The Sense of the Past, p. 196