I actually believe there's layers, and each layer is built on something different.
I believe the thing we have the most direct access to is our own mind, our own thoughts, which is not concrete.
I believe our minds are implemented in physical reality, out of physical things, which are concrete.
I believe the physical things in our reality are a manifestation of some kind of computational or mathematical structure, which is not concrete.
Most "properties" people refer to are properties we mentally apply onto concrete things. However, at least some of the "properties" of physical things which are manifestations of this mathematical or computational structure are more than just mental attributes we choose to apply - they're really real properties.
I think there's a fundamental difference between these really real properties, and "properties" that we as humans like to label stuff with.