John Wilkins has an interesting post on similarity. Since this is closely related to my ideas about evolutionary epistemology, I am adding my two cents.
A great deal of philosophical explanation is based on similarity. For example, it is often said we apply names such as “cat” and “dog” based on the similarity to some sort of archetype that we are said to have in our heads. However, it is never explained how similarity works, nor how that archetype gets into our heads in the first place.
My own view is that philosophy has much of this almost exactly backwards. The view of philosophers is that organize the world by grouping or categorizing similar things together. My apparently heretical alternative view is that we organize the world according to whatever ways we can find that work to organize the world, and that prove useful to us. Of particular importance is that our ways of organizing the world must be reliable and repeatable and useful (pragmatic). That is to say, we organize the world in a way so that if we went back and did it again in the same way, we would come up with the same result, and we do it in order to make it easier for us to negotiate our way around our world.
Once we have a reliable way of organizing the world, then we can call things “similar” to the extent that our method of organization groups those things into common categories.
Allow me to redescribe this in terms of what I call “partitioning”.
What is immediately available to us, or immediately available to a newborn infant, is a world, but perhaps a world with no known structure. In order to better apprehend the world, we try to find reliable ways of dividing that world up into parts. For example, the newborn infant eventually learns how to partition the world into daytime and nighttime, and that ability is welcomed by parents when it finally allows them to get a good night’s sleep. Once we have partitioned, we can then further subdivide or partition those partitions into finder sub-partitions. The result is a scheme of nested partitioning. Then, the further down this hierarchy that two things are found in the same partition at that level, the more similar they will seem to us.
The overall point. Our idea of similarity is a consequence of how we organize the world, rather than the underlying principle that we use to carry out that organizing.