Yōga Sūtra 1.06

प्रमाणविपर्ययविकल्पनिद्रास्मृतयः।

They are: pramāna, viparyaya, vikalpa, nidra, and smritayaha.

When mind takes the form of activity, it can only be in one of the above five stated activities. They can be difficult and not so difficult.