statistical mechanics - Boltzmann equation for a collisionless medium?

In the derivation of the Boltzmann equation (link to Wikipedia) for a collisionless gas it is assumed that: $$f\left( \vec{r} + \frac{ \vec{p} }{m} \Delta t, \ \vec{p} + \vec{F} \Delta t, \ t + \Delta t \right) = f\left( \vec{r}, \vec{p}, t \right)$$

What is the reasoning for this expression.