Why did Jeremy Bentham argue that Blackstone confused the law as it existed with the law as it ought to be?

Jeremy Bentham argued that Sir William Blackstone, in his Commentaries on the Laws of England, conflated the questions of what the law is with what the law ought to be.1

Blackstone sought to systematize English common law by emphasizing the "wisdom" of its principles, suggesting that their long-standing acceptance and historical use granted them legitimacy and binding force. Bentham, however, believed this approach was fundamentally flawed. By treating existing common law as inherently wise and legitimate simply because it had been accepted over time, Bentham argued that Blackstone stifled necessary legal reform.1

Bentham contended that this confusion prevented the law from being updated to address the rapidly changing social and economic conditions of the late 18th century. For Bentham, failing to distinguish between descriptive law (what is) and normative law (what ought to be) hindered the ability to critically evaluate and improve legal systems to better serve society.1