Chapter 1, sutra 4: "Elsewhere, identification with the form and nature of vrttis."  Swami Veda Bharati
Explanation:  When the thought-waves, vrittis, are present in the mind, the Self-luminous Seer mistakenly identifies itself with them, i.e. with mental activity rather than with itself--the self-luminous self. So, it might be said that there is a tendency in our mind to mistakenly identify ourselves with things outside of our essence. As will be seen in subsequent sutras, this leads to suffering.