The Tree eventually spreads into forty twigs, but it branches out gradually: see the picture at left.

In every example, two or more things come together in one's mind. From the ground upward, the first split is into things that fit or things that match. The branch for things that fit spreads directly into six twigs, but the branch for things that match (it is really the main stem, not just a branch) makes a three-way split: there is a short barren stump with no label*, the main stem continuing on up (named "one mental and one external"), and a stout branch to the right named "both external". Suppose you see a dog and remember having seen it before; the memory is "in your head" whereas the dog is out there in the real world --- that is what is meant by one mental and one external. By contrast, when you see two socks and note that they form a pair, the two items are both "out there" or external; neither is mental in the way that a memory is, or a thing expected, or a thing imagined.

         The "both external" branch splits five ways, into sounds, actions, items in a row, items arranged symmetrically and loose items that simply look alike. The main stem splits four ways, according to whether the mental item is remembered, expected, imagined or intended. And where the mental item is imagined, we separate occasions where an item is imagined and then created (by a painter, a poet or a speaker) from occasions where the external object exists independently and promotes a mental response when observed; there is something already present in the viewer's mind, perhaps vague and subconscious, that makes the viewer think "Yes, that  captures what I vaguely feel".

     (There is a refinement here that readers may pursue if they wish.)

*The stump represents matching of two items that are "both mental". For example, one might hear two jokes and store them in memory. Perhaps a week later, mulling them over, one might notice that in basic structure, they are alike --- just different settings or narrative styles. The match that one notices is quite genuine, but slightly abstract or nebulous so I have confined attention to the main stem and right-hand branch.