Krabbe maintained that the binding force of the law is founded on the "legal consciousness" of mankind: a normative feeling inherent to human psychology.

His work is expressive of the progressive and cosmopolitan ideals of interwar internationalism, and his notion of "sovereignty of law" stirred up much controversy in the legal scholarship of the time.

[4] Under the direction of the minister Tak van Poortvliet (progressive liberals)[5][6] he played an important role in drafting a proposal for reform of the electoral system[2][4] that, had it been approved, would have extended the right to vote to all male citizens who could read and write and who were self-supporting.

Among his notable students at Leiden were Roelof Kranenburg (1880–1956), a constitutional lawyer and politician, and the economist Gijsbert Weijer Jan Bruins [nl].

[14][1] Three years after resigning as professor, he published his Kritische Darstellung der Staatslehre ("Critical presentation of the theory of the state") but soon retired as chairman of the "Vereeniging voor Wijsbegeerte des Rechts" (Association for the philosophy of law), which had been set up in 1919 partly on his initiative.

[1][16] Shortly before the start of World War I, Krabbe developed a theory of law and state that was destined to stir up much controversy in the interwar period.

Relatively close to Otto von Gierke, Hugo Preuss,[36] Leon Duguit,[37][38] Georges Scelle,[39] Léon Michoud [fr],[40] Krabbe's work exhibits a strong normative orientation and emphasis on progressive and cosmopolitan ideals, which make it an exemplar of a psychologically inspired natural law theory:[25] On this natural mental faculty [rechtsbewustzijn] rests the validity of all law.

[34] The English translation of the latter book was undertaken by two renowned American academics, the political philosopher Georg H. Sabine and the professor of political science Walter J. Shepard [de]; their eighty-page translators' introduction attests to Krabbe's international standing,[12] his topicality at the time and the salience of his contribution to the international liberal values during the interwar period often associated with Wilsonianism.

[48][49][50] These cosmopolitan and progressive ideas, typical of the interwar "peace-through-law movement",[b] were often received with incredulity by the legal scholar of the time:[12][1] Roelof Kranenburg, a loyal disciple, wrote that Krabbe had in him "as much of the prophet as of the professor".