Refinement
The point is that even with, say, two material items such as socks, it is my
mental image of
one that matches my mental image of the other. Long before I get any
spasm of pleasure, my vision system has done a lot of work on the first signals
from my retinas, transforming them as they work their way to the upper reaches
of my brain. In fact I have already unified my present perception of the first
sock with my
recollection of many socks from the past, or with my general concept of a sock;
the image of that sock is definitely mental. And so too is the image of
the second sock. It is after the second mental image has formed that something
develops in my brain from the resemblance or overlap between these two mental
images. The label "both external" on the picture of the Tree's
branches is conveniently terse but is over-simple.