Philosophy Class

"The Truth will make you odd."

               Principle of Non-contradiction

Before we attempt to understand any philosophy at all, we must understand Aristotle's Principle of Non-Contradiction.

Plato also believed in it, but did not present it in a formal manner.  Without this principle, no thinking is possible at all, about anything.



"The obvious, the silly, and the true had got to be defended..... With the feeling that he was speaking to O'Brien (Big Brother), and also that he was setting forth an important axiom, he wrote:

Freedom is the freedom to say that two plus two make four. If that is granted, all else follows.  (Editor's note: "2 plus 2 is four" is

 not "freely believed, it is necessity, but Big Brother wanted necessary conclusions to be "freely accepted" ones.)

( George Orwell, Nineteen Eighty Four, (Penguin) , p. 68 )

"He wrote first in clumsy capitals:


Then almost without a pause he wrote beneath it:


He accepted everything. The past was alterable. The past never had been altered..... Anything could be true......"


Aristotle says:

Certain people "..from lack of education, demand that this principle (the principle of non-contradiction) too (also) be demonstrated : for it is a lack of education not to know that it is necessary to seek demonstration of some propositions and not of others. For there cannot be a demonstration of everything altogether; there would then be an infinite regress, and hence there would still be no final demonstration. But if there are propositions of which it is necessary to seek a demonstration, then those who refuse to admit ours ought to say what principle they would rather accept." Aristotle, Metaphysics

A/K/A The Principal of Contradiction (a telescope of the term.)

A/K/A Law of consistency.

No system will operate if it lands itself in a contradiction. Also no system will operate if it lands itself up in an inconsistency, that is it is self refuting.

Ways to state this principle:

1. A being cannot be and not be at the time.

2. A thing either is or is not.

3. Whatever is, is.'

4. A being is what it is and is not what it is not.


Explain: Football analogy: The defense of a position is one solid block. and a good quarterback will find a hole in it...


In law you get a witness to contradict himself... Crook alibi theory: He says he was in the barroom at the time of the murder with his friends.  security videos show him at crime scene...


Scientist saw that the theory that planets went in circles contradicted observation, so he came up with theory of ellipses....


In one place Kant says outside world is created by mind. In another he says, "Things themselves cause our perceptions."... Which is it? It can't be both.


Interested in detective stories? A detective was called in when a bank was robbed. Evidence showed it was between 4:30 and 5:00 PM. The clerk he suspected at first, said he was at Joe's Happy Hour. He had witnesses to prove it, not to even mention security videos.  The clerk was crossed off the list of suspects.



Contraries are not contradictories. Contraries are different things within a class.

Example: Blue, green, brown, and yellow are contraries within the genus of color.

The contradictory of color is non-color.


Animals, plants, humans are in the types of living things. Aquinas, following his mentor Aristotle, said that sometimes, even

though there are things that are clearly in the classes of plants, animals, etc., there are some things in which it is hard to

determine their class. He stated in essence the following:, "The lower point of the higher touches the higher point of the lower."  Most will be definitely known to be in the determinate class.


He also said that even though we can certainly tell the individuals of a species such as the individuals in the species of man,

there are some types of things in which we cannot distinguish the individual member of the species such as in certain types of plants.

In relation to those which are animals and those which are not: name 5 contraries in  animal. Name 2 contradictories.

Why did I ask for only 2 contradictories?