Now comparison is being described. The instrument of the valid knowledge of similarity is comparison. For instance a man who has seen a cow's form in cities and has gone to a forest, where his eyes have come in contact with a gayal (gavaya - bos gaurus) has the cognition, "This thing is like a cow". Then he has the conviction, "My cow is like this." Here by a process of agreement and difference, the knowledge of the likeness of a cow which exists in a gayal is the instrument, and the knowledge of that likeness of a gayal which exists in a cow is the result.
this is not possible through perception, for then the cow's form is not in contact with the eyes. Nor is it possible through inference, for that likeness of a cow which exists in a gayal cannot be the sign (reason) for inferring the likeness of a gayal in a cow. Nor can it be urged that this is possible through the following inference:
My cow is like this gayal.
Because it corresponds to its likeness existing in a gayal.
That which corresponds to its likeness existing in a thing is like the latter.
As Caitra, who corresponds to his likeness existing in Maitra, is like him.
For even without this sort of inference, the cognition, "My cow is like this," is a matter of common experience, and has also the apperception, "I am comparing the two,". Hence comparison is a distinct means of knowledge.
The first thing that must be remarked is the insistence that upamana is a distinct means of knowledge which is to say that it cannot be reduced to anything more basic or primitive. It thus takes its place along with the others such as Perception and Inference. There are 6 in all accepted within the philosophical system known as Advaitic Vedanta. It would take us too far out of our way to go into any detail about the rest of them though later I may offer a note on anupalabadhi or non-apprehension of existence.
Let me first make the point that perhaps 'comparison' may not be quite the apposite word in relation to this means of knowledge. I will be staying with the canonical example of the gayal for the moment. When the cow is seen in the gayal where is there what we would strictly in English call a comparison? When one speaks of comparison one generally requires that there be two things, a eye-witness description and a known individual. The one is compared to the other and a match is declared or a partial match or a mismatch. This seeing of the cow in the gayal seems to me to more like the successful deployment of the concept 'cow'. Our having the concept 'cow' allows us to 'see' the cow in the gayal. There is no comparison as such, having the concept is what enables us to do this, to have this recognition so to speak. For this basic reason I would stay with the term upamana to avoid what I think is an erroneous identification with the notion of comparison.
In Mental Acts Peter Geach speaks of concepts as capacities exercised in acts of judgement. Does exercising the concept just involve 'seeing' the cow in the gayal? Is the notion of comparison a misunderstanding that the translation of upamana underlines? We can construe what we have done as comparison only after the fact. That is what we must be doing we say to ourselves presenting an analysis that makes sense to us. It makes sense to us because of the inherent hankering after empiricist explanations. Evidence is required even if we have to make it up to have a likely story.
Why do we find it hard to accept that there things that we can do, that there are powers we just have and concept acquisition and concept utilisation are those sort of powers or they are perhaps a single power of double aspect. Having the concept 'white' means among other things seeing it in milk and chalk. No comparisons needed.
I am I have to admit ignorant of later speculation about pramanas and specifically this one amongst that school known as the New Logicians (Navya Nyaya) in its later phase. The perennial focus on the gavaya (bos gaurus) by commentators in general leads me to think that the general understanding of what was at issue was not clear. The canonical example is often a lifebuoy to cling to in a sea of incomprehension. My own speculations may be a matter of my own fancy and no better.