Patanjali Yoga Sutra 4.21

Sanskrit Sutra

चितेरप्रतिसंक्रमायास्तदाकारापत्तौ स्वबुद्धि-संवेदनम्॥२१॥

Swami Vivekananda's translation

The essence of knowledge (the Purusha) being unchangeable, when the mind takes its form, it becomes conscious.

Chapter link: Yoga Sutra Chapter 4