The principle of excluded middle is another form of the principle of contradiction, implying the same general fact, and resting on the same foundation. It is, that of two contradictories both cannot be false, or one must be true. Any given assertion must be either true or false; either the affirmative is true, or otherwise the negative is true, which means that the affirmative is false. "This house is either mine or not mine;" "Gold is yellow, gold is not yellow," cannot be both false, one must be true. There is no middle cause in such an alternative. But on examination, it will appear that this prin ciple does not hold in the same unqualified sense as the principle of contradiction; for the attribute affirmed or denied must be something intelligible and definite, as well as relevant to the subject in hand. We often say such a thing is neither big nor little, implying that there is a certain mean point that excludes the extremes, and yet those two terms arc the negative of each other. In a word, it is an essential condition of the
principle that the universe of the proposition should be distinctly understood and kept in view. if we say, "This is either red or not red," the alternative is indisputable within the universe "color," but not otherwise; the taste of an orange is neither red nor not red; if we jump over the boundaries of the class, the principle no longer holds good.
The three principles of identity, contradiction, and excluded middle, are usually talked of as necessities of the human mind, from which there is no escape. But we have just seen that in the case of the excluded middle:there are possible evasions; and even the principle of contradiction itself is flatly met by Hegel, who lays it down as a maxim of his philosophy that " being" and " not being" are the same, and deduces important inferences therefrom. All this should make us cautious in declaring any formula or any doctrine to be absolutely necessary, or imperative on the human mind.