Law of noncontradiction
In order for something to be contradictory, it must violate the
law of noncontradiction. This law states that A cannot be both A (what it is) and non-A (what it is not) at the same time and in the same relationship. In other words, you have contradicted yourself if you affirm and deny the same statement.
For example, if I say that the moon is made entirely of cheese but then also say that the moon is not made entirely of cheese, I have contradicted myself.
- The Law of Non-contradiction (QuickTime), by Ron Nash
Other statements may at first seem contradictory but are really not. Theologian R.C. Sproul cites as an example Dickens' famous line, “It was the best of times, it was the worst of times.” Obviously this is a contradiction if Dickens means that it was the best of times in the same way that it was the worst of times. But he avoids contradiction with this statement because he means that in one sense it was the best of times, but in another sense it was the worst of times.
- "Does God Observe the Law of Contradiction? ... Should We?" (PDF), by Richard L. Pratt, Jr.