Below are two images. At right, is a painting, from around 1750, of Radha, Krishna's beloved. At left, is a photograph I took in 2017, of a lotus flower floating in a pond (in Nara, Japan). What similarities are there between them?
This question is harder to answer than it seems at first. From childhood, we are trained to identify similarities. When I was young, similarity and difference were themes on Sesame Street, Highlights magazine and other "educational entertainment." As we get older, we use similarity in increasingly complex ways. On the basis of two things because similar in some respect, we can reason by analogy. Perhaps you get a worrying insect bite while hiking and look at some pictures online. On the basis of the bite on your arm being similar to the ones you see, you conclude that you were bitten by the same creature. We use similarity in scientific reasoning. For instance, I construct a model of how I think the world works. If my model is similar enough to the actual world, I can draw conclusions about the world from what I find in my model.
But explaining similarity itself--what it is and how we can recognize it--is difficult. Kumārila Bhaṭṭa, whose Tantravārttika I have been posting about recently, appeals to similarity in his explanation of how secondary meaning works, when it involves similar properties (gauṇavṛtti). This means we need to look to his understanding of similarity (sādṛśya) for a fuller picture of the account. Discussion of similarity occurs in a few places, one of the most important being his discussion of upamāna, often translated as "analogy."
In this section Kumārila takes on Naiyāyikas and Buddhists both. Against the Buddhists, he argues that similarity is a vastu, or a "positive entity" (as Jha translates it). He says:
However, similarity's being an object (vastu) cannot be denied (na śakyam apabādhitum).
That similarity is the presence of (bhūyaḥ) a connection (yogaḥ) with common constituent parts (avayava-sāmānya) which belongs to another universal (jāty-antarasya).
Now, something's having similarity of constituent parts is when the lotus-petal (padma-dala) is like the eye (akṣivat).
That similarity would occur on the basis of (bhūmnā) the commonality of having constituent parts (svâvayava-sāmānya)which are in the other [universal].
sādṛśyasya api vastutvaṃ na śakyam apabādhitum |
bhūyo 'vayava-sāmānya-yogo jāty-antarasya tat || 18 ||
sadṛśâvayavatvaṃ tu yatra padma-dalâkṣivat |
tat svâvayava-sāmānya-bhūmnā teṣā[ṃ] bhaviṣyati || 19 ||
|The sarpagandha plant|
|The ketakīgandha plant (I think!)|
One question I'm considering, after Roy Perrett asked me about it after a talk on Kumārila's theory of gauṇavṛtti, is the relationship between similarity as analytic and Indian philosophers conceive of the ideas. Similarity is typically taken to be symmetric, reflexive, and transitive by analytic philosophers. But I'm not yet sure whether Kumārila would commit to reflexivity, since he seems to characterize similarity as depending on difference, and at the smallest atomic point, there is no similarity (vs 28). But an atomic point could be self-similar, for analytic philosophers.
Unfortunately, to my knowledge, very little has been written on this, or it's not in English or languages I can read--Uma Chattopadhyay's books on upamāna only dip briefly into this section and while Kei Kataoka wrote something a while ago, it's only in Japanese, as far as I know. But I'll keep an eye out. Those few people (!) who read this blog, I'd be happy to hear suggestions for secondary scholarship.