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Comments, contributions, thoughts, musings, etc are welcome and appreciated.

## What are these pages?

Here are resources on ( thinking / knowledge / calculation ) which is ( back of the envelope / rough and ready / order of magnitude / thinking on your feet / efficacious / numerate / approximate ) .

Standing on the back of the envelope, one can see much of the universe. I have found it a vantage point of great power and beauty, but unfortunately one much neglected. It is my hope, in collecting these resources, to increase its accessibility, and to draw greater attention to it.

- Mitchell N Charity <mcharity@lcs.mit.edu>

It is rather remarkable how much we can understand our world. But the opportunity is often obscured. Either by sacrificing numeracy for simplicity, and finding both lost. Or by failing to sacrifice trivia for understanding. But there is a middle path. One of models and math both simple and powerful. Of thoughtfully chosen, `good enough' precision. Of knowledge, live and insightful, which reveals rather than obscures. A path, and a view, from the back of the envelope.

## Current objectives

Encourage the use of the back of the envelope,

In particular, support the memes

• BotE is desirable.
• BotE is a good way to develop deep knowledge.
• BotE is accessible to young children.
• To support BotE, introductory material should have certain properties.
These pages are currently strongly biased towards physics. I believe BotE's applicability to be much more general. I have a couple of history pages in mind to illustrate.

## Motivation

Having some free time, I wanted to improve my understanding of the world. Brushing up my understanding of order of magnitude calculation seemed a good place to start. But I didn't find a good description online. But no problem, as teaching is a great way to learn something deeply, I set out to create one. As I pushed into the paper literature, and broadened my scope to back of the envelope work in general, I became increasingly bemused. It seemed that the back of the envelope was even more delightful, powerful, accessible to children, encouraging of deep understanding; even more of an all around neat thing than I had realized. There were scattered columns in various fields saying, this is good, we should more of it. But it still doesn't seem common. I have found it difficult to find introductions and resources. Perhaps another example of neat stuff currently being transmitted mainly by osmosis / apprenticeship.

It being difficult for people to use what is difficult to find, this then is a collection of introductions and resources.

As seems to so often happen, you set out to do something simple and straight-forward, but can't seem to find the tools you need, and thus end up in the tool creation business.

## Who am I?

I've spent the last few years working on the architecture of global-scale information systems, at MIT's Laboratory for Computer Science, my salary coming from your tax dollars via DARPA. I have moved on to consulting, but needed a vacation, and A View from the Back of the Envelope has been part of it.
- Mitchell N Charity <mcharity@lcs.mit.edu>

## Future

These pages cover thought, and some bare-hands observation (but nothing which requires having built tools), but little subsequent action. One can imagine a follow-on, A View from the Engineer's Workbench, with tool building and environment observation and manipulation, targeted at kids and everyone. Stuff like E&M coke bottle viewers, child-built "temporary" furniture, engineering with stuff around the house, expanding your observation/manipulation envelope, force transmission, etc etc etc). But that is yet another big hunk to bite off, so first I'll see how this one goes down.

## Thanks

Thanks go to the various people who have written columns and books (and web pages) with a BotE flavor. To an MIT professor, whose name escapes me, who taught a brief IAP seminar some years ago on Wise Approximate GuesseS (WAGS). To the librarians of the Minuteman Library Network, and Boston Library Consortium. One still needs to spend money to get a decent literature search capability, but it has been a great resource.
Specific thanks for suggestions and encouragement to: Suzanne Barricelli, Alan Bawden, Carolyn Breen, David Luongo, Philip and Phylis Morrison.
These pages are hosted by Vendian Systems.

 A View from the Back of the Envelope Comments encouraged. - Mitchell N Charity

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