In an indirect table, the attempt is to make each premise true and the conclusion false. If this can be done, then the argument is invalid. If it cannot be done, then the argument is valid.
Moving around an indirect table on the computer screen is very much like moving around a regular table except that a truth value entered from the keyboard may be erased by placing the cursor on it and pressing delete or the spacebar.
In the text inconsistent or contradictory assignments have an oval placed around them. On the computer screen these are flagged within exclamation points which are made by placing the cursor on any portion of the inconsistent argument part and pressing F6. A second pressing of F6 removes the exclamation points. These commands are also available from the Indirect Truth Table Menu.
Multiple cases are generated by the computer when F8 is pressed. Move the cursor to the premise or to the conclusion from which the cases are to be made and press F8. These commands are also available from the Indirect Truth Table Menu.
The computer generates more cases than are shown in the text.
Suppose C ≡ B is one of the premises and so the goal is to make it true.
| TEXT | COMPUTER | ||
| C ≡ B | C≡ B | ||
| |T| | |T| | ||
| |T| | ! |F| ! | ||
| ! |F| ! | |||
| |T| |
The extra cases will always contradict the goal truth value for the Main Operator of that part of the argument. Here the false rows can immediately be marked by moving the cursor to them and pressing F6. The individual truth values generated by the cases cannot be erased. Press F8 again to delete all LINES but the highlighted one. Now the VALUES can be erased.