Full Idea
Primitivism is the view that time has a direction, and that its having that direction is intrinsic to time itself. Reductionism is the view that time has a direction, but its having that direction is reducible to something else.
Gist of Idea
The direction of time is either primitive, or reducible to something else
Source
Baron,S/Miller,K (Intro to the Philosophy of Time [2019], 5.3.1)
Book Reference
Baron,S/Miller,K: 'Introduction to the Philosophy of Time' [Polity 2019], p.130
A Reaction
The general suggestion for the second theory is that time's direction reduces to some aspect of the laws of nature. I strongly incline to the primitive view. Something's got to be primitive.