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9/7/2026

The code for Deriver and Derivation Planner was written for the Mac around 1988 by Martin Frické using THINK Pascal. Those applications were published in 1989. Martin Frické, their author, variously converted these to different host environments. Somewhat more recently, around 2008 and later, those conversions were to java applications and applets, and to javascript widgets. Often the Google Web Toolkit (GWT) was used to compile java to javascript. Almost all of these projects have been 'open-sourced' and are available free on GIT.

9/6/2026

The fallacy, in English

9/8/2026

The fallacy, in English

"Everything has a cause. Therefore, something is the cause of everything." The first sentence is a reasonable premise. The second does not follow from it — and mixing them up is a genuinely common move (it turns up, for instance, in causal versions of the cosmological argument). Logicians call this the *quantifier-shift fallacy*: swapping the order of `∀` and `∃` changes what is being claimed.

9/9/2026

The fallacy, in English (illicit conversion)

'All cats are mammals' is true. It does not follow that 'all mammals are cats'. The inference is obviously invalid, yet the same move with something less concrete ('all F are G, so all G are F') is a standing trap. The moment the example is less obviously wrong than cats and mammals, trouble lurks.

9/7/2026

The first fallacy, in English

"If it rained, the ground would be wet. The ground is wet. So it rained." This feels right — and is wrong. The ground could be wet for some other reason. This is one of the oldest and most persistent fallacies in ordinary reasoning: affirming the consequent. 

The inference here is of the kind

R → W, W ∴ R

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8/1/26

The program, widgets, or Notes, should be accompanied by a suitable textbook, such as:

M.Bergmann, J.Moor, J.Nelson, The Logic Book
A.Hausman, H.Kahane, P.Tidman, Logic and Philosophy 
W.Hodges, Logic
C.Howson, Logic with Trees 
R.C.Jeffrey, Formal Logic: Its Scope and Limits
H.Leblanc and W.Wisdom, Deductive Logic
B.Mates, Elementary Logic
M.D.Resnick, Elementary Logic

8/15/26

Roll your own

• You have to use the right (unicode/html) logical symbols. Check Writing symbols

• The symbols in use here are ~ & ∨ →  

  ≡

8/15/26

Reading

A text like

Rod Girle [2000] Modal Logics and Philosophy

would definitely be a help here.

Tutorial

[Modal logic is a vast area, what is being presented here is the briefest of glimpses through the shop window (a book like the Girle would help you go further).]

In modal logic, there are the additional symbols □ (necessary) and ◊ (possible) and, in the background, the notion of 'possible worlds'.

Last altered 6/20/12

A central use for Trees is to produce a counter example to an invalid argument. To do this, you construct a tree with a complete open branch. You will be able to do this for invalid arguments (but not valid ones). Then you run up that branch assigning all atomic formulas True and all negations of atomic formulas False.

 

This software will let you try a few.

 

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Exercise: Finding a counter-example.