T5.6 9 of 9

Good. We now have the tools for complex derivations. The last demonstration suggested two things.

  1. When an accessible line has main connective tilde, you may well have to reiterate that statement within a subderivation to provide a contradiction.
  2. When all else fails or when you begin to feel desperate, you may have to assume the negation of your goal and work toward a contradiction

A last word of advice: To get good at derivations, you'll need lots of practice on the harder ones requiring some strategy. If they seem too difficult at first just redo some problems you've seen done in this tutorial. Or do problems (like those in the link below) with lots of hints. Later, see if you can do these completely on your own.

Finally, a for making your way to the end of this long tutorial...


In 5.6 exercises, your computer will do much of the busy work of derivations for you. All you do is type in line numbers and justification as usual, press TAB, and enter '/' or '//'. The result will appear automatically!

Password: To use 5.6ex V, you'll need to enter "77" at the prompt. Remember, the password is "77".

So, you may use 5.6ex V and let the computer do your work. For example, if you have a long premise on line 1 and another long one on line 2, and need to conjoin them, just write:

1,2 &I

press TAB, type '/', TAB and the computer will do all the rest of the typing for you. (Just don't forget "77"!) Oh, and if you type '//' instead, you'll get a different, correct result. Try it and see.

Unadvertised Secret: this '/' or '//' trick will work for some of 5.5 and all exercises in 5.6 and beyond! Just don't tell anyone...


Time to start doing problems.
Take me to the next tutorial!
Back to chapter five, please.