As the Roman republic grew and then transformed into an empire, its rulers faced the increasing challenge of governing an evermore diverse and far-flung population. Legal questions and disputes inevitably arose not only among Roman citizens, but with non-citizens living in or traveling through its territories, to whom the ius civile did not apply. This led to the development of the ius gentium ("law of nations") and ius naturale ("natural law"). The ius gentium, or law of nations/peoples, was the body of laws that applied to all people, foreigners and non-citizens as well as citizens, and was based upon the common principles and reasoning that civilized societies and humankind were understood to live by and share. These laws common to all people were further understood to be rooted in the ius naturale, or natural law, a category of law based on the principles shared by all living creatures, humans as well as animals (such as laws pertaining to procreation, or physical defense against attack).