Chapter 1, Sutra 9: "Dependent upon a verbal knowledge only but devoid of a real object is the vrtti called imaginary cognition, vikalpa." Swami Veda Bharati
Explanation: When a cognition arises in the mind from words that describe a thing which does not actually exist, it is called vikalpa. Vikalpa might also be called fantasy, imagination, and linguistic misconception. Unlike viparyaya, the subject of the previous Sutra 1:8, which has no use, vikalpa is often useful in that it is the basis of fictional writing, and is often used to describe the indescribable, such as when it is said "Divinity is formless, never born, never dies, etc," in that the words describe something that does not exist and so are vikalpa. When yogis, through meditation, seek to know the truth of reality, they must let go of vikalpa. Yogis seek to know through direct experience, not through a belief system.