Sada jnatascitta-vrttayas tat-prabhoh purusasyaparinamitvat.
Sada = always; jnata = known; citta-vrttayah = modifications of mind; tat-prabhoh = of its lord; purusasya = of purusa; Aparinamitvat = on account of the changelessness.
The modifications of the mind are always known to its lord on account of the changelessness of the Purusa. (IV-18)
There is a subject-object relationship between various kosas (sheaths) of our system. The changes in body are known to mind when mind is subject and is its object of observation. Similarly our emotional upsurges etc become the object of observation for the intellect. Body, Prana, Mind, Intellect they all keep changing constantly. There is one changeless entity i.e. pure consciousness or Purusa which witnesses various vehicles (kosas) including Vijnanamaya Kosa, the intellect. Purusa is the ultimate subjective entity witnessing all the changes in various kosas including Manomaya (mind).
All the modifications of the mind are even 'known' to Purusa. This entity called Purusa is the connecting medium for all other entities. And it is because of this unchanging entity that we are able to recollect even the events of our childhood. One can go back in the past and recollect it because all that takes place in the mind is known to its lord, the Purusa.
It is because of the unchanging nature of consciousness which acts like a screen (constant base) for moving images that all the changes in the mind are 'known' to it.