Full Idea
In Newton's laws of motion time is squared, so a negative value gives the same result as a positive value, which means Newtonian mechanics cannot distinguish between the two directions of time.
Gist of Idea
Newtonian mechanics does not distinguish negative from positive values of time
Source
report of Isaac Newton (Principia Mathematica [1687]) by P Coveney / R Highfield - The Arrow of Time 2 'anatomy'
Book Reference
Coveney,P/Highfield R: 'The Arrow of Time' [Flamingo 1990], p.54
A Reaction
Maybe Newton just forgot to mention that negative values were excluded. (Or was he unaware of the sequence of negative integers?). Too late now - he's done it.