I have just begun my journey in Classical Mechanics with the book "An Introduction to Mechanics" by Kleppner and Kolenkow. I find myself stuck at Newton's first law. The book presents Newton's first law as follows: "Newton's first law is an assertion that inertial frames exist." The book also states: "Newton's first law is part definition and part experimental. Isolated bodies move uniformly in inertial systems is by virtue of the definition of an inertial system. In contrast, the assertion that inertial systems exist is a statement about the physical world." The above statements lead me to believe that inertial systems exist. However, it is believed that an inertial system does not exist. Does this belief not contradict the statements above? Do inertial frames really exist? Also, in an inertial frame, not only isolated bodies but any body experiencing a net zero force moves with constant velocity( or moves uniformly ). This is the first law that we all know. The book, however, focuses only on isolated bodies. Does this not leave us with an incomplete understanding of the first law?