Table 8
| p | q | p + q - pq |
| 1 | 1 | 1 + 1 - 1 = 1 |
| 0 | 1 | 0 + 1 - 0 = 1 |
| 1 | 0 | 1 + 0 - 0 = 1 |
| 0 | 0 | 0 + 0 - 0 = 0 |
Now we know that P or Q is true if and only if either one or both of P and Q are true, and thus we see that the calculation is correct.
The algebra of logic (see also Supplement 2) is a more efficient way of calculating logical truth. But it is still a good deal of work to use the algebra. For example, if we have 10 conditions, we shall have 10 letters like p, q to handle in calculations. Thus we need a still more efficient way.