Full Idea
The 'must' of Newton's First Law is different. There is no deeper level relative to the processes described to give a mechanism which generates uniform motion. There is no such mechanism. ..It specifies what it is for something to be a material thing.
Gist of Idea
The necessity of Newton's First Law derives from the nature of material things, not from a mechanism
Source
Rom Harré (Laws of Nature [1993], 4)
Book Reference
Harré,Rom: 'Laws of Nature' [Duckworth 1993], p.100
A Reaction
Harré says the law can only exist as part of a network of other ideas.