They were not, indeed, formally and exactly set forth in a single written instrument; but they were to be found scattered over our ancient and noble statutes; and, what was of far greater moment, they had been engraven on the hearts of Englishmen during four hundred years.
That, without the consent of the representatives of the nation, no legislative act could be passed, no tax imposed, no regular soldiery kept up, that no man could be imprisoned, even for a day, by the arbitrary will of the sovereign, that no tool of power could plead the royal command as a justification for violating any right of the humblest subject, were held, both by Whigs and Tories, to be fundamental laws of the realm.

A realm of which these were the fundamental laws stood in no need of a new constitution.
But, though a new constitution was not needed, it was plain that changes were required.

The misgovernment of the Stuarts, and the troubles which that misgovernment had produced, sufficiently proved that there was somewhere a defect in our polity; and that defect it was the duty of the Convention to discover and to supply.
Some questions of great moment were still open to dispute.

Our constitution had begun to exist in times when statesmen were not much accustomed to frame exact definitions.

Anomalies, therefore, inconsistent with its principles and dangerous to its very existence, had sprung up almost imperceptibly, and, not having, during many years, caused any serious inconvenience, had gradually acquired the force of prescription.