Value pluralism holds that human values are genuinely plural, sometimes incompatible, and not reducible to any single master value. Liberty and equality, mercy and justice, loyalty and honesty can conflict, and when they do, something of real worth is lost whichever way the conflict is settled.
Isaiah Berlin is its most important modern defender, and the claim is stronger than it first appears. It is not that we lack the information to rank values, nor that people happen to disagree about them. It is that the values themselves do not form a single ordered system, so the idea of a perfect society realizing all genuine goods at once is not merely unattainable but incoherent. Weber had described something similar as the polytheism of modern life. Bernard Williams and Joseph Raz developed the position in different directions.
Berlin drew a liberal conclusion. If no single conception of the good can be shown to be the true one, the case for coercing people into it collapses, and a politics of trade-off, compromise and preserved options is the honest response. This is also where his warning about positive liberty lands: utopian projects have been most dangerous when convinced that the values they sacrificed were not really values at all.
Whether the liberal conclusion follows is the sharpest argument inside the tradition. John Gray has pressed the case that it does not. If values are genuinely incommensurable, then liberalism is one form of life among others and not the uniquely rational response to pluralism, which makes Berlin's universal claim for it inconsistent with his own premise. Berlin's defenders answer that pluralism is not relativism, since the range of genuine human values is wide but bounded and cruelty and domination fall outside it. That reply is more persuasive at the extremes than in the difficult middle.