Patanjali Yoga Sutra 4.17

Sanskrit Sutra

सदा ज्ञाताश्चित्तवृत्तयस्तत्प्रभोः पुरुषस्यापरिणामित्वात्॥१७॥

Swami Vivekananda's translation

The states of the mind are always known, because the lord of the mind, the Purusha, is unchangeable.

Chapter link: Yoga Sutra Chapter 4