One possibility is that over the long term our conversation will come to resemble a thicket of trees. That is, new topics will arise and then quickly branch out, and the branches will generate more branches, until at last the topic peters out.

By definition, a new topic will begin with an independent voice card, that is, a voice card with no down arrow in the first browsing box. Often we won't realize that a card is the start of something big until several more volumes have accumulated.

If this first card generates three responses, those three cards can be seen as nodes branching off the original "trunk." Of course each of those nodes may generate a new set of branches. Every card that fails to generate a further response is like a leaf at the top of the tree.

Imagine a diagram of this tree which starts with the trunk node at the bottom of the page. Each new volume adds a higher level of nodes to the tree with clumps of nodes springing from individual nodes in the previous level. Eventually the nodes peter out near the top of the page.

Taken as a whole, this tree structure comprises a single topic. Each volume of voice cards is a cross section in the temporal development of the topic tree. When we use the arrow buttons to move back and forth through a particular volume, we are exploring a cross section, a frozen moment in the development of a related set of ideas. At any point we can use the down arrows to work our way back in time, eventually arriving at the trunk. Or we can use the reply box icons to move forward in time out to the myriad leaves at the edge of the tree. The pointing hand buttons can be used to explore local clumps of nodes.

When you look at in this way, the left and right arrows are for "spatial browsing" and the down arrow and reply icons are for "temporal browsing." Of course new topic trees will be rising and fading all the time and each volume will contain a whole thicket of trees with branches tangled all together.

One of the most remarkable properties of speaking through cards is that the conversation is self-documenting, that is, we will automatically accumulate a permanent record as we go. If we can keep up a conversation for a decade or more, we will have quite a remarkable forest of topic trees on our hands (as well as a severe shortage of disk space).

From time to time, if all goes well, I will issue retrospectives. These will consist of all the voice cards pertaining to a single topic, gathered from a whole series of previous volumes and arranged in a single stack. These will, in effect, be single trees that I have disentangled from the larger thicket.

At this point, all of this is sheer speculation. But I have always enjoyed climbing in trees, and I look forward to the time when we can all climb together!