The essence of knowledge (the Purusha) being unchangeable, when the mind takes its form, it becomes conscious.
In plain words: The Purusha, the true Self, never changes. When the mind takes on its reflection, the mind becomes aware and seems to be the Self for a while.
Patanjali says this to make it more clear that knowledge is not a quality of the Purusha. When the mind comes near the Purusha it is reflected, as it were, upon the mind, and the mind, for the time being, becomes knowing and seems as if it were itself the Purusha.
Namaste.
A calm guide to life, the mind and the old teachings.