Legal realism is a naturalistic approach to law; it is the view that jurisprudence should emulate the methods of natural science; that is, it should rely on empirical evidence.
Locating the meaning of law in places such as legal opinions issued by judges and their deference to or dismissal of precedent and the doctrine of stare decisis, it stresses the importance of understanding the factors involved in judicial decision-making.
When realists such as Oliver Wendell Holmes Jr. pointed out that individuals embroiled in the legal system generally wanted to know what was going to happen, Hart assumed that they were offering the necessary and sufficient conditions for the use of the concept of "law."
American legal realism has aptly been described as "the most important indigenous jurisprudential movement in the United States during the twentieth century".
In the early years of the twentieth century, formalist approaches to the law had been forcefully criticized by thinkers such as Roscoe Pound, John Chipman Gray, and Benjamin Cardozo.
Philosophers such as John Dewey had held up empirical science as a model of all intelligent inquiry, and argued that law should be seen as a practical instrument for advancing human welfare.
Outside the realm of law, in fields such as economics and history, there was a general "revolt against formalism," a reaction in favor of more empirical ways of doing philosophy and the human sciences.
The felt necessities of the time, the prevalent moral and political theories, intuitions of public policy, avowed or unconscious, and even the prejudices which judges share with their fellow-men, have had a good deal more to do than the syllogism in determining the rules by which men should be governed.
The law embodies the story of a nation's development through many centuries, and it cannot be dealt with as if it contained only the axioms and corollaries of a book of mathematics.
For the bad man, "legal duty" signifies only "a prophecy that if he does certain things he will be subjected to disagreeable consequences by way of imprisonment or compulsory payment".
"[12] Before the Civil War, this conception of adjudication as a form of social engineering had been widely shared by American judges, but in the late nineteenth century it had fallen out of favor.
Among the leading legal realists were Karl Llewellyn, Jerome Frank, Herman Oliphant, Underhill Moore, Walter Wheeler Cook, Leon Green, and Felix Cohen.
As Hart notes, this completely misses the fact that judges use legal rules to guide their decisions, not as data to predict their eventual holdings.
[18] Other critics, such as Ronald Dworkin and Lon Fuller, have faulted legal realists for their attempt to sharply separate law and morality.