Blackstone conflated the legal and the moral, implying that English law was rational because it was English law. Bentham separates these entirely: a law's existence is one question, its merit is another. Existing law can be described without endorsing it. This separation is the precondition for critical jurisprudence — for asking, systematically and honestly, whether current laws serve human happiness and, if not, how they should be reformed.
Once law is understood as a human construct, it follows that the legislator's task is scientific and forward-looking. Rather than interpreting a received tradition, the rational legislator must calculate the effects of proposed rules on human happiness and design institutions accordingly. Bentham's entire reform programme — from prison design to procedural law to colonial administration — follows from this reconception of the legislator as an engineer of social utility rather than a guardian of inherited wisdom.
The distinction between expository and censorial jurisprudence (law as it is vs law as it ought to be) is introduced in the Fragment's Preface and elaborated throughout the Introduction.