Assimilation (upamana) is the instrument of assimilative cognition. Assimilative Cognition (upamiti) consists in the of the relation between a name and the object denoted by it. Knowledge of similarity is the efficient instrument of such (karana} cognition.(from A Primer of Indian Logic
This may be illustrated thus :
A person happens to be ignorant of the exact meaning of the word gavaya (a particular animal of the bovine species). From a forester, he learns that a gavaya is similar to a cow;; he goes to a forest sees the animal called gavaya, which is similar to a cow and recollects the information conveyed by the assimilative proposition.
Then the assimilative cognition, "This is the animal (of the bovine species) denoted by the word gavaya' arises.Thus ends the chapter on upamana.
ACCORDING TO ANNAMBHATTA'S TARKASAMGRAHA translated by S.KUPPUSWAMI SASTRI )
I’m beginning to have an obsession with the gavaya similar in scope to Aunt Betsy Trotwood’s one with the donkeys in David Copperfield. I want to drive them out, I want a gavaya free account of similarity, I want to strip back to the bare wood before all the fancy finishes were applied. Let’s look at it free from the Nyaya version which puts denotation at its centre. Kuppuswami Sastri tells us that this was because they wished to distinguish it from the pramana of anumana or inference.
The Nyaya conception of upamna as a distinct instrument of valid cognition restricts its scope to ascertainment of the denotative or primary significative power of a word (saktigraha). The chief object of the Naiyayikas in so restricting its scope is to save it
from being swallowed up in inference (anumana).
In saving it in this way they missed, I think, the fundamental nature of the judgment this is like x or alternatively this is unlike x or this is the same x that I told you about. What makes this pramana( i.e. means of valid knowledge not reducible to any other), unique and fundamental is that the concept of ‘like’ cannot be taught: it is an immediate intuition. From the further elaboration of this native power denotation may be discovered. Am I saying that it is something that you just ‘see’ and is therefore perception. No, I mean it involves perception plus a judgment.
For the moment the gavayas are out of the garden and back in the forest again.