You've probably heard the story...
The Eastern guru, holy lice in his beard and all, affirms that the earth is supported on the back of a tiger.
When asked what supports the tiger, he says it stands upon an elephant.
When asked what supports the elephant he says it is a giant turtle.
When asked, finally, what supports the giant turtle, he is briefly taken aback ... but quickly replies "Ah, after that it's turtles all the way down."
(According my son Zebulon, after a few billion turtles, one comes to an enchilada, and after that it's enchiladas all the way down -- but that's the same basic idea.)
The Peircean view of the universe as Third is much like this amusing parable.
In the realm of Third, it's patterns all the way down!
There are patterns ... but these patterns must be patterns of arrangement of something, of some "substrate."
But of what does this substrate consist? This substrate only enters into the realm of Third insofar as it presents itself as a set of observed patterns.
But, patterns in what?
These must be patterns in some substrate.
Or is it patterns all the way down?!
In the realm of science, the recursion ends with observations that are commonly accepted within some community. Some Master Dataset is accepted by a community as constituting valid observations, and then patterns are recognized in this dataset.
But the recognition of these commonly accepted observations as patterns in individual sense-data -- this is where the "patterns all the way down" bottomless recursion is pushed, in the case of science.
You never reach a solid reality whose existence is just known ... not in the realm of Third. Because the only thing that is known is relationship, and a relationship must be a relationship among some entities. But if these entities are part of the realm of Third, then ... etc.
First and Second, in a sense, might seem to bottom out the recursion -- but they don't really, because they are a different order of being.
The nature of life is that we just keep discovering relations among relations among relations ... and relations beneath relations beneath relations ... and never get to a fundamental reality underneath. We may get to things that seem fundamental -- but how can we ever really know?
This fundamental bottomlessness of the realm of pattern and relationship underlies a lot of other issues that will occur a little later in these pages, including the possibility that our reality is in some sense a "simulation", and the notion of the universe as a multiverse or multi-multi-...-multiverse.
Turtles on turtles on ... turtles on turtles; patterns in patterns ... in patterns in patterns.
Mathematically one can only model this kind of recursion using obscure constructs like hypersets.
But experientially they are not hard to appreciate, if one avoids being caught up in "naive realist" perspectives that hold there is some absolute reality bottoming out the hierarchy/heterarchy of relationships.