Nyāya Sūtras 24

Then the definition of doubt, which indeed has an occasion, must be stated; so it is stated:

NyS 1.1.23: Doubt is a wavering cognition that attends to a specific feature, arising from the apprehension of common properties or manifold properties, from conflicting opinion, or from the irregularity of apprehension or non-apprehension.

Doubt is a wavering cognition that attends to a specific feature, arising from the apprehension of a common property. Seeing in a post and a person their common property of height and girth, and desiring to know their previously observed specific feature, one does not determine which one it is, wondering 'what could it be?'; that non-determining cognition is doubt. The cognition that 'I apprehend their common property, but do not apprehend the specific feature of either' is the attentiveness
that prompts doubt; therefore doubt is a wavering cognition that attends to a specific feature.
'Arising from the apprehension of manifold properties': 'manifold' means both what belongs to the same class and what belongs to a different class. From the apprehension of properties of that manifold—

because a specific feature is observed in both ways. Objects are distinguished from things belonging to the same class and from things belonging to a different class: earth, by virtue of having smell, is distinguished from water and so forth, and from qualities and actions. And in sound, being produced by disjunction is a specific feature. Regarding it, there is doubt whether it is a substance, a quality, or an action, because a specific feature is observed in both ways.

Is it a distinguishing property of an existent substance apart from qualities and actions, or of an existent quality, or of an existent action? The expectation of a specific property is the cognition: “I do not apprehend any property that definitively establishes one of these.”
Because of contradiction: Contradiction is the observation of conflicting views on a single object; conflict means opposition, the inability to coexist. One view is: “The self exists”; another is: “The self does not exist.” And both existence and non-existence cannot possibly obtain in one and the same place together, nor is a reason establishing either of the two apprehended; there, the failure to ascertain reality is doubt.
Moreover, because of the irregularity of apprehension: real water is apprehended in ponds and the like, and non-existent water is apprehended in mirages. Therefore, when something is apprehended somewhere, because of the non-apprehension of a means of valid cognition that definitively establishes its reality, the doubt arises: “Is what is apprehended real or non-existent?”
Because of the irregularity of non-apprehension: a real thing is not apprehended, such as roots, hidden stakes, underground water, and the like, and a non-existent thing, such as what is not yet produced or what has ceased, is also not apprehended.

Therefore, when something is not apprehended somewhere, doubt arises: “Is what is not apprehended real or non-existent?” The expectation of a specific property is as before.
The previous [causes]—a common property and a unique property—reside in the object to be known, whereas apprehension and non-apprehension reside in the knower; it is by this specific difference that they are stated separately.

Doubt is deliberation depending on the recollection of a specific property arising from the apprehension of a common property, because of the possibility of a common property. //23