The tool of reflectivity from the coarse to the fine utilizes a concept of reflection, where as one can take that which is physical and coarse, and therefore more knowable and reflect upon that which is less coarse and less seeable. To understand this tool of reflectivity is to understand how to move through the house from the known to the unknown.
In the case of The Two-Story House, one establishes a law or an aspect of the Physical that is knowable and/or seeable and reflects upward (inward) to the Mental Mind, which is less coarse and less seeable. This is done much like one does when one steps up to a mirror with his physical body in order to see his image and determine certain aspects about it.
So, to investigate the Mental Mind, one must first investigate the Physical and establish some agreed upon laws or aspects about the Physical then use reflection to investigate. For example, my physical body requires food to live and the reflection is that the Mental Mind requires "food" as well.