What do you imagine under dynamic medium? Is there the dynamic medium, or are there more of them? What aspects of it are the most important? What tools will be used to interact with it (what are the analogs of pencils)? In what respect is it a medium －what does it mediate, and between whom? What kind of literacy will it enable?

I consider a dynamic medium to be any medium for expressing ideas that can respond to actions or changes made by the recipient of the message. This includes but is not limited to face-to-face conversations, spreadsheets, video games, text-based games, and most forms of simulation that the user can control or direct, such as physics or chemistry or biology simulations. There are many of them. You interact with them using whatever input and output devices the computer and the simulation support, including but not limited to keyboard, mouse, video, microphone, light pen, touchscreen, camera, game controller, VR headset, and position sensors. It mediates any idea that one human wishes to communicate to another, and is especially useful for ideas that involve systems that change over time. For example, spreadsheets are often used to simulate and communicate information about projected earnings and expenses. One person creates a spreadsheet that embodies a certain model of how the organization works and certain predictions about what is likely to happen. They then give it to another person, who can explore the model and see what it predicts if different choices are made or different events occur in the near future. What type of literacy it supports depends on the medium. Spreadsheets have probably done a lot to improve financial literacy. Video games have helped many people to understand how virtual combat systems work. (Whether or not that is useful information to know is beyond the scope of this comment.) Physics simulations help people to better understand Newtonian physics in an intuitive way, and to make fewer Aristotelian assumptions. Ultimately, once we have a dynamic medium that anyone can use to express their ideas, meaning a personal dynamic medium, it may support deeper thinking about processes and systems in general. Personally, although I don't think it will happen in our lifetime, I like to hope that it will ultimately make us smarter all the way around and help us to understand and solve our really big problems, like poverty, pollution, corruption, and war.

“Mojo of the thing is _simulation_” - Alan Kay

@Personal Dynamic Media Interesting... What do you mean by "the ideas can respond to interactions of the recipient"? Does that mean that eg. the spreadsheet is the idea, or does that mean that there is an agent/mechanism that updates the ideas/facts? Would this agent itself be a kind of medium?

In the case of a spreadsheet, the spreadsheet software is the medium. The idea is whatever you are encoding in a particular spreadsheet workbook using the medium. For example, your model of how finances work at your company and how they will be affected by your projections for income and expenses. Responding to the recipient means after you give me your workbook containing your model and your projections, I can make changes and observe the results. I can ask "what if" questions by checking to see what happens if our earnings end up being 30% lower than you project, for example.

@Personal Dynamic Media Would a video qualify as a dynamic medium? Certainly a spreadsheet has a richer interaction scheme; can one somehow measure the richness of interactions provided by the medium? For example, a spreadsheet that allows to "compute backwards" (set the result and observe the possible compatible input configurations) could be richer than a regular spreadsheet. Is there then a "maximally rich" dynamic medium?

@Jan Ruzicka In my opinion, the typical video recording is not a dynamic medium. You cannot ask it "what if" questions or run it with a different set of data or assumptions. A video call is a dynamic medium because you can explain something and I can ask you questions about it. I don't know if there is one maximally dynamic medium, or if what constitutes being maximally dynamic might vary based on domain, or if there is even a limit to how dynamic a medium can be. In general, I think there is a consensus that any dynamic medium not involving direct personal interaction gives the person creating a message the ability to program, and gives the person receiving the message the ability to run that program. Ideally, it also gives the recipient the ability to modify the program. There is also, in general, a belief that ideally modifications can be made while the program is being run, without having to change into a separate authoring mode. Beyond those general principles, there is a lot of variation in opinions about the best way to let the user manipulate, observe, and interact with the message/simulation/program.